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postgres/doc/src/sgml/plpgsql.sgml
Tom Lane e3e3d2a789 Extend ExecMakeFunctionResult() to support set-returning functions that return 
via a tuplestore instead of value-per-call.  Refactor a few things to reduce
ensuing code duplication with nodeFunctionscan.c.  This represents the
reasonably noncontroversial part of my proposed patch to switch SQL functions
over to returning tuplestores.  For the moment, SQL functions still do things
the old way.  However, this change enables PL SRFs to be called in targetlists
(observe changes in plperl regression results).
2008-10-28 22:02:06 +00:00

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<!-- $PostgreSQL: pgsql/doc/src/sgml/plpgsql.sgml,v 1.135 2008/10/28 22:02:05 tgl Exp $ -->
<chapter id="plpgsql">
<title><application>PL/pgSQL</application> - <acronym>SQL</acronym> Procedural Language</title>
<indexterm zone="plpgsql">
<primary>PL/pgSQL</primary>
</indexterm>
<sect1 id="plpgsql-overview">
<title>Overview</title>
<para>
<application>PL/pgSQL</application> is a loadable procedural
language for the <productname>PostgreSQL</productname> database
system. The design goals of <application>PL/pgSQL</> were to create
a loadable procedural language that
<itemizedlist>
<listitem>
<para>
can be used to create functions and trigger procedures,
</para>
</listitem>
<listitem>
<para>
adds control structures to the <acronym>SQL</acronym> language,
</para>
</listitem>
<listitem>
<para>
can perform complex computations,
</para>
</listitem>
<listitem>
<para>
inherits all user-defined types, functions, and operators,
</para>
</listitem>
<listitem>
<para>
can be defined to be trusted by the server,
</para>
</listitem>
<listitem>
<para>
is easy to use.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Functions created with <application>PL/pgSQL</application> can be
used anywhere that built-in functions could be used.
For example, it is possible to
create complex conditional computation functions and later use
them to define operators or use them in index expressions.
</para>
<sect2 id="plpgsql-advantages">
<title>Advantages of Using <application>PL/pgSQL</application></title>
<para>
<acronym>SQL</acronym> is the language <productname>PostgreSQL</>
and most other relational databases use as query language. It's
portable and easy to learn. But every <acronym>SQL</acronym>
statement must be executed individually by the database server.
</para>
<para>
That means that your client application must send each query to
the database server, wait for it to be processed, receive and
process the results, do some computation, then send further
queries to the server. All this incurs interprocess
communication and will also incur network overhead if your client
is on a different machine than the database server.
</para>
<para>
With <application>PL/pgSQL</application> you can group a block of
computation and a series of queries <emphasis>inside</emphasis>
the database server, thus having the power of a procedural
language and the ease of use of SQL, but with considerable
savings of client/server communication overhead.
</para>
<itemizedlist>
<listitem><para> Extra round trips between
client and server are eliminated </para></listitem>
<listitem><para> Intermediate results that the client does not
need do not have to be marshaled or transferred between server
and client </para></listitem>
<listitem><para> Multiple rounds of query
parsing can be avoided </para></listitem>
</itemizedlist>
<para> This can result in a considerable performance increase as
compared to an application that does not use stored functions.
</para>
<para>
Also, with <application>PL/pgSQL</application> you can use all
the data types, operators and functions of SQL.
</para>
</sect2>
<sect2 id="plpgsql-args-results">
<title>Supported Argument and Result Data Types</title>
<para>
Functions written in <application>PL/pgSQL</application> can accept
as arguments any scalar or array data type supported by the server,
and they can return a result of any of these types. They can also
accept or return any composite type (row type) specified by name.
It is also possible to declare a <application>PL/pgSQL</application>
function as returning <type>record</>, which means that the result
is a row type whose columns are determined by specification in the
calling query, as discussed in <xref linkend="queries-tablefunctions">.
</para>
<para>
<application>PL/pgSQL</> functions can be declared to accept a variable
number of arguments by using the <literal>VARIADIC</> marker. This
works exactly the same way as for SQL functions, as discussed in
<xref linkend="xfunc-sql-variadic-functions">.
</para>
<para>
<application>PL/pgSQL</> functions can also be declared to accept
and return the polymorphic types
<type>anyelement</type>, <type>anyarray</type>, <type>anynonarray</type>,
and <type>anyenum</>. The actual
data types handled by a polymorphic function can vary from call to
call, as discussed in <xref linkend="extend-types-polymorphic">.
An example is shown in <xref linkend="plpgsql-declaration-aliases">.
</para>
<para>
<application>PL/pgSQL</> functions can also be declared to return
a <quote>set</> (or table) of any data type that can be returned as
a single instance. Such a function generates its output by executing
<command>RETURN NEXT</> for each desired element of the result
set, or by using <command>RETURN QUERY</> to output the result of
evaluating a query.
</para>
<para>
Finally, a <application>PL/pgSQL</> function can be declared to return
<type>void</> if it has no useful return value.
</para>
<para>
<application>PL/pgSQL</> functions can also be declared with output
parameters in place of an explicit specification of the return type.
This does not add any fundamental capability to the language, but
it is often convenient, especially for returning multiple values.
The <literal>RETURNS TABLE</> notation can also be used in place
of <literal>RETURNS SETOF</>.
</para>
<para>
Specific examples appear in
<xref linkend="plpgsql-declaration-aliases"> and
<xref linkend="plpgsql-statements-returning">.
</para>
</sect2>
</sect1>
<sect1 id="plpgsql-structure">
<title>Structure of <application>PL/pgSQL</application></title>
<para>
<application>PL/pgSQL</application> is a block-structured language.
The complete text of a function definition must be a
<firstterm>block</>. A block is defined as:
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
<optional> DECLARE
<replaceable>declarations</replaceable> </optional>
BEGIN
<replaceable>statements</replaceable>
END <optional> <replaceable>label</replaceable> </optional>;
</synopsis>
</para>
<para>
Each declaration and each statement within a block is terminated
by a semicolon. A block that appears within another block must
have a semicolon after <literal>END</literal>, as shown above;
however the final <literal>END</literal> that
concludes a function body does not require a semicolon.
</para>
<tip>
<para>
A common mistake is to write a semicolon immediately after
<literal>BEGIN</>. This is incorrect and will result in a syntax error.
</para>
</tip>
<para>
A <replaceable>label</replaceable> is only needed if you want to
identify the block for use
in an <literal>EXIT</> statement, or to qualify the names of the
variables declared in the block. If a label is given after
<literal>END</>, it must match the label at the block's beginning.
</para>
<para>
All key words are case-insensitive.
Identifiers are implicitly converted to lowercase
unless double-quoted, just as they are in ordinary SQL commands.
</para>
<para>
There are two types of comments in <application>PL/pgSQL</>. A double
dash (<literal>--</literal>) starts a comment that extends to the end of
the line. A <literal>/*</literal> starts a block comment that extends to
the next occurrence of <literal>*/</literal>. Block comments cannot be
nested, but double dash comments can be enclosed into a block comment and
a double dash can hide the block comment delimiters <literal>/*</literal>
and <literal>*/</literal>.
</para>
<para>
Any statement in the statement section of a block
can be a <firstterm>subblock</>. Subblocks can be used for
logical grouping or to localize variables to a small group
of statements. Variables declared in a subblock mask any
similarly-named variables of outer blocks for the duration
of the subblock; but you can access the outer variables anyway
if you qualify their names with their block's label. For example:
<programlisting>
CREATE FUNCTION somefunc() RETURNS integer AS $$
&lt;&lt; outerblock &gt;&gt;
DECLARE
quantity integer := 30;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 30
quantity := 50;
--
-- Create a subblock
--
DECLARE
quantity integer := 80;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 80
RAISE NOTICE 'Outer quantity here is %', outerblock.quantity; -- Prints 50
END;
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 50
RETURN quantity;
END;
$$ LANGUAGE plpgsql;
</programlisting>
</para>
<note>
<para>
There is actually a hidden <quote>outer block</> surrounding the body
of any <application>PL/pgSQL</> function. This block provides the
declarations of the function's parameters (if any), as well as some
special variables such as <literal>FOUND</literal> (see
<xref linkend="plpgsql-statements-diagnostics">). The outer block is
labeled with the function's name, meaning that parameters and special
variables can be qualified with the function's name.
</para>
</note>
<para>
It is important not to confuse the use of
<command>BEGIN</>/<command>END</> for grouping statements in
<application>PL/pgSQL</> with the similarly-named SQL commands
for transaction
control. <application>PL/pgSQL</>'s <command>BEGIN</>/<command>END</>
are only for grouping; they do not start or end a transaction.
Functions and trigger procedures are always executed within a transaction
established by an outer query &mdash; they cannot start or commit that
transaction, since there would be no context for them to execute in.
However, a block containing an <literal>EXCEPTION</> clause effectively
forms a subtransaction that can be rolled back without affecting the
outer transaction. For more about that see <xref
linkend="plpgsql-error-trapping">.
</para>
</sect1>
<sect1 id="plpgsql-declarations">
<title>Declarations</title>
<para>
All variables used in a block must be declared in the
declarations section of the block.
(The only exceptions are that the loop variable of a <literal>FOR</> loop
iterating over a range of integer values is automatically declared as an
integer variable, and likewise the loop variable of a <literal>FOR</> loop
iterating over a cursor's result is automatically declared as a
record variable.)
</para>
<para>
<application>PL/pgSQL</> variables can have any SQL data type, such as
<type>integer</type>, <type>varchar</type>, and
<type>char</type>.
</para>
<para>
Here are some examples of variable declarations:
<programlisting>
user_id integer;
quantity numeric(5);
url varchar;
myrow tablename%ROWTYPE;
myfield tablename.columnname%TYPE;
arow RECORD;
</programlisting>
</para>
<para>
The general syntax of a variable declaration is:
<synopsis>
<replaceable>name</replaceable> <optional> CONSTANT </optional> <replaceable>type</replaceable> <optional> NOT NULL </optional> <optional> { DEFAULT | := } <replaceable>expression</replaceable> </optional>;
</synopsis>
The <literal>DEFAULT</> clause, if given, specifies the initial value assigned
to the variable when the block is entered. If the <literal>DEFAULT</> clause
is not given then the variable is initialized to the
<acronym>SQL</acronym> null value.
The <literal>CONSTANT</> option prevents the variable from being assigned to,
so that its value remains constant for the duration of the block.
If <literal>NOT NULL</>
is specified, an assignment of a null value results in a run-time
error. All variables declared as <literal>NOT NULL</>
must have a nonnull default value specified.
</para>
<para>
A variable's default value is evaluated and assigned to the variable
each time the block is entered (not just once per function call).
So, for example, assigning <literal>now()</literal> to a variable of type
<type>timestamp</type> causes the variable to have the
time of the current function call, not the time when the function was
precompiled.
</para>
<para>
Examples:
<programlisting>
quantity integer DEFAULT 32;
url varchar := 'http://mysite.com';
user_id CONSTANT integer := 10;
</programlisting>
</para>
<sect2 id="plpgsql-declaration-aliases">
<title>Aliases for Function Parameters</title>
<para>
Parameters passed to functions are named with the identifiers
<literal>$1</literal>, <literal>$2</literal>,
etc. Optionally, aliases can be declared for
<literal>$<replaceable>n</replaceable></literal>
parameter names for increased readability. Either the alias or the
numeric identifier can then be used to refer to the parameter value.
</para>
<para>
There are two ways to create an alias. The preferred way is to give a
name to the parameter in the <command>CREATE FUNCTION</command> command,
for example:
<programlisting>
CREATE FUNCTION sales_tax(subtotal real) RETURNS real AS $$
BEGIN
RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
</programlisting>
The other way, which was the only way available before
<productname>PostgreSQL</productname> 8.0, is to explicitly
declare an alias, using the declaration syntax
<synopsis>
<replaceable>name</replaceable> ALIAS FOR $<replaceable>n</replaceable>;
</synopsis>
The same example in this style looks like:
<programlisting>
CREATE FUNCTION sales_tax(real) RETURNS real AS $$
DECLARE
subtotal ALIAS FOR $1;
BEGIN
RETURN subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
</programlisting>
</para>
<note>
<para>
These two examples are not perfectly equivalent. In the first case,
<literal>subtotal</> could be referenced as
<literal>sales_tax.subtotal</>, but in the second case it could not.
(Had we attached a label to the block, <literal>subtotal</> could
be qualified with that label, instead.)
</para>
</note>
<para>
Some more examples:
<programlisting>
CREATE FUNCTION instr(varchar, integer) RETURNS integer AS $$
DECLARE
v_string ALIAS FOR $1;
index ALIAS FOR $2;
BEGIN
-- some computations using v_string and index here
END;
$$ LANGUAGE plpgsql;
CREATE FUNCTION concat_selected_fields(in_t sometablename) RETURNS text AS $$
BEGIN
RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
END;
$$ LANGUAGE plpgsql;
</programlisting>
</para>
<para>
When a <application>PL/pgSQL</application> function is declared
with output parameters, the output parameters are given
<literal>$<replaceable>n</replaceable></literal> names and optional
aliases in just the same way as the normal input parameters. An
output parameter is effectively a variable that starts out NULL;
it should be assigned to during the execution of the function.
The final value of the parameter is what is returned. For instance,
the sales-tax example could also be done this way:
<programlisting>
CREATE FUNCTION sales_tax(subtotal real, OUT tax real) AS $$
BEGIN
tax := subtotal * 0.06;
END;
$$ LANGUAGE plpgsql;
</programlisting>
Notice that we omitted <literal>RETURNS real</> &mdash; we could have
included it, but it would be redundant.
</para>
<para>
Output parameters are most useful when returning multiple values.
A trivial example is:
<programlisting>
CREATE FUNCTION sum_n_product(x int, y int, OUT sum int, OUT prod int) AS $$
BEGIN
sum := x + y;
prod := x * y;
END;
$$ LANGUAGE plpgsql;
</programlisting>
As discussed in <xref linkend="xfunc-output-parameters">, this
effectively creates an anonymous record type for the function's
results. If a <literal>RETURNS</> clause is given, it must say
<literal>RETURNS record</>.
</para>
<para>
Another way to declare a <application>PL/pgSQL</application> function
is with <literal>RETURNS TABLE</>, for example:
<programlisting>
CREATE FUNCTION extended_sales(p_itemno int) RETURNS TABLE(quantity int, total numeric) AS $$
BEGIN
RETURN QUERY SELECT quantity, quantity * price FROM sales WHERE itemno = p_itemno;
END;
$$ LANGUAGE plpgsql;
</programlisting>
This is exactly equivalent to declaring one or more <literal>OUT</>
parameters and specifying <literal>RETURNS SETOF
<replaceable>sometype</></literal>.
</para>
<para>
When the return type of a <application>PL/pgSQL</application>
function is declared as a polymorphic type (<type>anyelement</type>,
<type>anyarray</type>, <type>anynonarray</type>, or <type>anyenum</>),
a special parameter <literal>$0</literal>
is created. Its data type is the actual return type of the function,
as deduced from the actual input types (see <xref
linkend="extend-types-polymorphic">).
This allows the function to access its actual return type
as shown in <xref linkend="plpgsql-declaration-type">.
<literal>$0</literal> is initialized to null and can be modified by
the function, so it can be used to hold the return value if desired,
though that is not required. <literal>$0</literal> can also be
given an alias. For example, this function works on any data type
that has a <literal>+</> operator:
<programlisting>
CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement)
RETURNS anyelement AS $$
DECLARE
result ALIAS FOR $0;
BEGIN
result := v1 + v2 + v3;
RETURN result;
END;
$$ LANGUAGE plpgsql;
</programlisting>
</para>
<para>
The same effect can be had by declaring one or more output parameters as
polymorphic types. In this case the
special <literal>$0</literal> parameter is not used; the output
parameters themselves serve the same purpose. For example:
<programlisting>
CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement,
OUT sum anyelement)
AS $$
BEGIN
sum := v1 + v2 + v3;
END;
$$ LANGUAGE plpgsql;
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-declaration-type">
<title>Copying Types</title>
<synopsis>
<replaceable>variable</replaceable>%TYPE
</synopsis>
<para>
<literal>%TYPE</literal> provides the data type of a variable or
table column. You can use this to declare variables that will hold
database values. For example, let's say you have a column named
<literal>user_id</literal> in your <literal>users</literal>
table. To declare a variable with the same data type as
<literal>users.user_id</> you write:
<programlisting>
user_id users.user_id%TYPE;
</programlisting>
</para>
<para>
By using <literal>%TYPE</literal> you don't need to know the data
type of the structure you are referencing, and most importantly,
if the data type of the referenced item changes in the future (for
instance: you change the type of <literal>user_id</>
from <type>integer</type> to <type>real</type>), you might not need
to change your function definition.
</para>
<para>
<literal>%TYPE</literal> is particularly valuable in polymorphic
functions, since the data types needed for internal variables can
change from one call to the next. Appropriate variables can be
created by applying <literal>%TYPE</literal> to the function's
arguments or result placeholders.
</para>
</sect2>
<sect2 id="plpgsql-declaration-rowtypes">
<title>Row Types</title>
<synopsis>
<replaceable>name</replaceable> <replaceable>table_name</replaceable><literal>%ROWTYPE</literal>;
<replaceable>name</replaceable> <replaceable>composite_type_name</replaceable>;
</synopsis>
<para>
A variable of a composite type is called a <firstterm>row</>
variable (or <firstterm>row-type</> variable). Such a variable
can hold a whole row of a <command>SELECT</> or <command>FOR</>
query result, so long as that query's column set matches the
declared type of the variable.
The individual fields of the row value
are accessed using the usual dot notation, for example
<literal>rowvar.field</literal>.
</para>
<para>
A row variable can be declared to have the same type as the rows of
an existing table or view, by using the
<replaceable>table_name</replaceable><literal>%ROWTYPE</literal>
notation; or it can be declared by giving a composite type's name.
(Since every table has an associated composite type of the same name,
it actually does not matter in <productname>PostgreSQL</> whether you
write <literal>%ROWTYPE</literal> or not. But the form with
<literal>%ROWTYPE</literal> is more portable.)
</para>
<para>
Parameters to a function can be
composite types (complete table rows). In that case, the
corresponding identifier <literal>$<replaceable>n</replaceable></> will be a row variable, and fields can
be selected from it, for example <literal>$1.user_id</literal>.
</para>
<para>
Only the user-defined columns of a table row are accessible in a
row-type variable, not the OID or other system columns (because the
row could be from a view). The fields of the row type inherit the
table's field size or precision for data types such as
<type>char(<replaceable>n</>)</type>.
</para>
<para>
Here is an example of using composite types. <structname>table1</>
and <structname>table2</> are existing tables having at least the
mentioned fields:
<programlisting>
CREATE FUNCTION merge_fields(t_row table1) RETURNS text AS $$
DECLARE
t2_row table2%ROWTYPE;
BEGIN
SELECT * INTO t2_row FROM table2 WHERE ... ;
RETURN t_row.f1 || t2_row.f3 || t_row.f5 || t2_row.f7;
END;
$$ LANGUAGE plpgsql;
SELECT merge_fields(t.*) FROM table1 t WHERE ... ;
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-declaration-records">
<title>Record Types</title>
<synopsis>
<replaceable>name</replaceable> RECORD;
</synopsis>
<para>
Record variables are similar to row-type variables, but they have no
predefined structure. They take on the actual row structure of the
row they are assigned during a <command>SELECT</> or <command>FOR</> command. The substructure
of a record variable can change each time it is assigned to.
A consequence of this is that until a record variable is first assigned
to, it has no substructure, and any attempt to access a
field in it will draw a run-time error.
</para>
<para>
Note that <literal>RECORD</> is not a true data type, only a placeholder.
One should also realize that when a <application>PL/pgSQL</application>
function is declared to return type <type>record</>, this is not quite the
same concept as a record variable, even though such a function might
use a record variable to hold its result. In both cases the actual row
structure is unknown when the function is written, but for a function
returning <type>record</> the actual structure is determined when the
calling query is parsed, whereas a record variable can change its row
structure on-the-fly.
</para>
</sect2>
<sect2 id="plpgsql-declaration-renaming-vars">
<title><literal>RENAME</></title>
<synopsis>
RENAME <replaceable>oldname</replaceable> TO <replaceable>newname</replaceable>;
</synopsis>
<para>
Using the <literal>RENAME</literal> declaration you can change the
name of a variable, record or row. This is primarily useful if
<varname>NEW</varname> or <varname>OLD</varname> should be
referenced by another name inside a trigger procedure. See also
<literal>ALIAS</literal>.
</para>
<para>
Examples:
<programlisting>
RENAME id TO user_id;
RENAME this_var TO that_var;
</programlisting>
</para>
<note>
<para>
<literal>RENAME</literal> appears to be broken as of
<productname>PostgreSQL</> 7.3. Fixing this is of low priority,
since <literal>ALIAS</literal> covers most of the practical uses
of <literal>RENAME</literal>.
</para>
</note>
</sect2>
</sect1>
<sect1 id="plpgsql-expressions">
<title>Expressions</title>
<para>
All expressions used in <application>PL/pgSQL</application>
statements are processed using the server's main
<acronym>SQL</acronym> executor. For example, when you write
a <application>PL/pgSQL</application> statement like
<synopsis>
IF <replaceable>expression</replaceable> THEN ...
</synopsis>
<application>PL/pgSQL</application> will evaluate the expression by
feeding a query like
<synopsis>
SELECT <replaceable>expression</replaceable>
</synopsis>
to the main SQL engine. While forming the <command>SELECT</> command,
any occurrences of <application>PL/pgSQL</application> variable names
are replaced by parameters, as discussed in detail in
<xref linkend="plpgsql-var-subst">.
This allows the query plan for the <command>SELECT</command> to
be prepared just once and then reused for subsequent
evaluations with different values of the variables. Thus, what
really happens on first use of an expression is essentially a
<command>PREPARE</> command. For example, if we have declared
two integer variables <literal>x</> and <literal>y</>, and we write
<programlisting>
IF x &lt; y THEN ...
</programlisting>
what happens behind the scenes is
<programlisting>
PREPARE <replaceable>statement_name</>(integer, integer) AS SELECT $1 &lt; $2;
</programlisting>
and then this prepared statement is <command>EXECUTE</>d for each
execution of the <command>IF</> statement, with the current values
of the <application>PL/pgSQL</application> variables supplied as
parameter values.
The query plan prepared in this way is saved for the life of the database
connection, as described in
<xref linkend="plpgsql-plan-caching">. Normally these details are
not important to a <application>PL/pgSQL</application> user, but
they are useful to know when trying to diagnose a problem.
</para>
</sect1>
<sect1 id="plpgsql-statements">
<title>Basic Statements</title>
<para>
In this section and the following ones, we describe all the statement
types that are explicitly understood by
<application>PL/pgSQL</application>.
Anything not recognized as one of these statement types is presumed
to be an SQL command and is sent to the main database engine to execute,
as described in <xref linkend="plpgsql-statements-sql-noresult">
and <xref linkend="plpgsql-statements-sql-onerow">.
</para>
<sect2 id="plpgsql-statements-assignment">
<title>Assignment</title>
<para>
An assignment of a value to a <application>PL/pgSQL</application>
variable or row/record field is written as:
<synopsis>
<replaceable>variable</replaceable> := <replaceable>expression</replaceable>;
</synopsis>
As explained above, the expression in such a statement is evaluated
by means of an SQL <command>SELECT</> command sent to the main
database engine. The expression must yield a single value.
</para>
<para>
If the expression's result data type doesn't match the variable's
data type, or the variable has a specific size/precision
(like <type>char(20)</type>), the result value will be implicitly
converted by the <application>PL/pgSQL</application> interpreter using
the result type's output-function and
the variable type's input-function. Note that this could potentially
result in run-time errors generated by the input function, if the
string form of the result value is not acceptable to the input function.
</para>
<para>
Examples:
<programlisting>
tax := subtotal * 0.06;
my_record.user_id := 20;
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-statements-sql-noresult">
<title>Executing a Command With No Result</title>
<para>
For any SQL command that does not return rows, for example
<command>INSERT</> without a <literal>RETURNING</> clause, you can
execute the command within a <application>PL/pgSQL</application> function
just by writing the command.
</para>
<para>
Any <application>PL/pgSQL</application> variable name appearing
in the command text is replaced by a parameter symbol, and then the
current value of the variable is provided as the parameter value
at run time. This is exactly like the processing described earlier
for expressions; for details see <xref linkend="plpgsql-var-subst">.
As an example, if you write:
<programlisting>
DECLARE
key TEXT;
delta INTEGER;
BEGIN
...
UPDATE mytab SET val = val + delta WHERE id = key;
</programlisting>
the command text seen by the main SQL engine will look like:
<programlisting>
UPDATE mytab SET val = val + $1 WHERE id = $2;
</programlisting>
Although you don't normally have to think about this, it's helpful
to know it when you need to make sense of syntax-error messages.
</para>
<caution>
<para>
<application>PL/pgSQL</application> will substitute for any identifier
matching one of the function's declared variables; it is not bright
enough to know whether that's what you meant! Thus, it is a bad idea
to use a variable name that is the same as any table, column, or
function name that you need to reference in commands within the
function. For more discussion see <xref linkend="plpgsql-var-subst">.
</para>
</caution>
<para>
When executing a SQL command in this way,
<application>PL/pgSQL</application> plans the command just once
and re-uses the plan on subsequent executions, for the life of
the database connection. The implications of this are discussed
in detail in <xref linkend="plpgsql-plan-caching">.
</para>
<para>
Sometimes it is useful to evaluate an expression or <command>SELECT</>
query but discard the result, for example when calling a function
that has side-effects but no useful result value. To do
this in <application>PL/pgSQL</application>, use the
<command>PERFORM</command> statement:
<synopsis>
PERFORM <replaceable>query</replaceable>;
</synopsis>
This executes <replaceable>query</replaceable> and discards the
result. Write the <replaceable>query</replaceable> the same
way you would write an SQL <command>SELECT</> command, but replace the
initial keyword <command>SELECT</> with <command>PERFORM</command>.
<application>PL/pgSQL</application> variables will be
substituted into the query just as for commands that return no result,
and the plan is cached in the same way. Also, the special variable
<literal>FOUND</literal> is set to true if the query produced at
least one row, or false if it produced no rows (see
<xref linkend="plpgsql-statements-diagnostics">).
</para>
<note>
<para>
One might expect that writing <command>SELECT</command> directly
would accomplish this result, but at
present the only accepted way to do it is
<command>PERFORM</command>. A SQL command that can return rows,
such as <command>SELECT</command>, will be rejected as an error
unless it has an <literal>INTO</> clause as discussed in the
next section.
</para>
</note>
<para>
An example:
<programlisting>
PERFORM create_mv('cs_session_page_requests_mv', my_query);
</programlisting>
</para>
</sect2>
<sect2 id="plpgsql-statements-sql-onerow">
<title>Executing a Query with a Single-Row Result</title>
<indexterm zone="plpgsql-statements-sql-onerow">
<primary>SELECT INTO</primary>
<secondary>in PL/pgSQL</secondary>
</indexterm>
<indexterm zone="plpgsql-statements-sql-onerow">
<primary>RETURNING INTO</primary>
<secondary>in PL/pgSQL</secondary>
</indexterm>
<para>
The result of a SQL command yielding a single row (possibly of multiple
columns) can be assigned to a record variable, row-type variable, or list
of scalar variables. This is done by writing the base SQL command and
adding an <literal>INTO</> clause. For example,
<synopsis>
SELECT <replaceable>select_expressions</replaceable> INTO <optional>STRICT</optional> <replaceable>target</replaceable> FROM ...;
INSERT ... RETURNING <replaceable>expressions</replaceable> INTO <optional>STRICT</optional> <replaceable>target</replaceable>;
UPDATE ... RETURNING <replaceable>expressions</replaceable> INTO <optional>STRICT</optional> <replaceable>target</replaceable>;
DELETE ... RETURNING <replaceable>expressions</replaceable> INTO <optional>STRICT</optional> <replaceable>target</replaceable>;
</synopsis>
where <replaceable>target</replaceable> can be a record variable, a row
variable, or a comma-separated list of simple variables and
record/row fields.
<application>PL/pgSQL</application> variables will be
substituted into the rest of the query, and the plan is cached,
just as described above for commands that do not return rows.
This works for <command>SELECT</>,
<command>INSERT</>/<command>UPDATE</>/<command>DELETE</> with
<literal>RETURNING</>, and utility commands that return row-set
results (such as <command>EXPLAIN</>).
Except for the <literal>INTO</> clause, the SQL command is the same
as it would be written outside <application>PL/pgSQL</application>.
</para>
<tip>
<para>
Note that this interpretation of <command>SELECT</> with <literal>INTO</>
is quite different from <productname>PostgreSQL</>'s regular
<command>SELECT INTO</command> command, wherein the <literal>INTO</>
target is a newly created table. If you want to create a table from a
<command>SELECT</> result inside a
<application>PL/pgSQL</application> function, use the syntax
<command>CREATE TABLE ... AS SELECT</command>.
</para>
</tip>
<para>
If a row or a variable list is used as target, the query's result columns
must exactly match the structure of the target as to number and data
types, or a run-time error
occurs. When a record variable is the target, it automatically
configures itself to the row type of the query result columns.
</para>
<para>
The <literal>INTO</> clause can appear almost anywhere in the SQL
command. Customarily it is written either just before or just after
the list of <replaceable>select_expressions</replaceable> in a
<command>SELECT</> command, or at the end of the command for other
command types. It is recommended that you follow this convention
in case the <application>PL/pgSQL</application> parser becomes
stricter in future versions.
</para>
<para>
If <literal>STRICT</literal> is not specified in the <literal>INTO</>
clause, then <replaceable>target</replaceable> will be set to the first
row returned by the query, or to nulls if the query returned no rows.
(Note that <quote>the first row</> is not
well-defined unless you've used <literal>ORDER BY</>.) Any result rows
after the first row are discarded.
You can check the special <literal>FOUND</literal> variable (see
<xref linkend="plpgsql-statements-diagnostics">) to
determine whether a row was returned:
<programlisting>
SELECT * INTO myrec FROM emp WHERE empname = myname;
IF NOT FOUND THEN
RAISE EXCEPTION 'employee % not found', myname;
END IF;
</programlisting>
If the <literal>STRICT</literal> option is specified, the query must
return exactly one row or a run-time error will be reported, either
<literal>NO_DATA_FOUND</> (no rows) or <literal>TOO_MANY_ROWS</>
(more than one row). You can use an exception block if you wish
to catch the error, for example:
<programlisting>
BEGIN
SELECT * INTO STRICT myrec FROM emp WHERE empname = myname;
EXCEPTION
WHEN NO_DATA_FOUND THEN
RAISE EXCEPTION 'employee % not found', myname;
WHEN TOO_MANY_ROWS THEN
RAISE EXCEPTION 'employee % not unique', myname;
END;
</programlisting>
Successful execution of a command with <literal>STRICT</>
always sets <literal>FOUND</literal> to true.
</para>
<para>
For <command>INSERT</>/<command>UPDATE</>/<command>DELETE</> with
<literal>RETURNING</>, <application>PL/pgSQL</application> reports
an error for more than one returned row, even when
<literal>STRICT</literal> is not specified. This is because there
is no option such as <literal>ORDER BY</> with which to determine
which affected row should be returned.
</para>
<note>
<para>
The <literal>STRICT</> option matches the behavior of
Oracle PL/SQL's <command>SELECT INTO</command> and related statements.
</para>
</note>
<para>
To handle cases where you need to process multiple result rows
from a SQL query, see <xref linkend="plpgsql-records-iterating">.
</para>
</sect2>
<sect2 id="plpgsql-statements-executing-dyn">
<title>Executing Dynamic Commands</title>
<para>
Oftentimes you will want to generate dynamic commands inside your
<application>PL/pgSQL</application> functions, that is, commands
that will involve different tables or different data types each
time they are executed. <application>PL/pgSQL</application>'s
normal attempts to cache plans for commands (as discussed in
<xref linkend="plpgsql-plan-caching">) will not work in such
scenarios. To handle this sort of problem, the
<command>EXECUTE</command> statement is provided:
<synopsis>
EXECUTE <replaceable class="command">command-string</replaceable> <optional> INTO <optional>STRICT</optional> <replaceable>target</replaceable> </optional> <optional> USING <replaceable>expression</replaceable> <optional>, ...</optional> </optional>;
</synopsis>
where <replaceable>command-string</replaceable> is an expression
yielding a string (of type <type>text</type>) containing the
command to be executed. The optional <replaceable>target</replaceable>
is a record variable, a row variable, or a comma-separated list of
simple variables and record/row fields, into which the results of
the command will be stored. The optional <literal>USING</> expressions
supply values to be inserted into the command.
</para>
<para>
No substitution of <application>PL/pgSQL</> variables is done on the
computed command string. Any required variable values must be inserted
in the command string as it is constructed; or you can use parameters
as described below.
</para>
<para>
Also, there is no plan caching for commands executed via
<command>EXECUTE</command>. Instead, the
command is prepared each time the statement is run. Thus the command
string can be dynamically created within the function to perform
actions on different tables and columns.
</para>
<para>
The <literal>INTO</literal> clause specifies where the results of
a SQL command returning rows should be assigned. If a row
or variable list is provided, it must exactly match the structure
of the query's results (when a
record variable is used, it will configure itself to match the
result structure automatically). If multiple rows are returned,
only the first will be assigned to the <literal>INTO</literal>
variable. If no rows are returned, NULL is assigned to the
<literal>INTO</literal> variable(s). If no <literal>INTO</literal>
clause is specified, the query results are discarded.
</para>
<para>
If the <literal>STRICT</> option is given, an error is reported
unless the query produces exactly one row.
</para>
<para>
The command string can use parameter values, which are referenced
in the command as <literal>$1</>, <literal>$2</>, etc.
These symbols refer to values supplied in the <literal>USING</>
clause. This method is often preferable to inserting data values
into the command string as text: it avoids run-time overhead of
converting the values to text and back, and it is much less prone
to SQL-injection attacks since there is no need for quoting or escaping.
An example is:
<programlisting>
EXECUTE 'SELECT count(*) FROM mytable WHERE inserted_by = $1 AND inserted <= $2'
INTO c
USING checked_user, checked_date;
</programlisting>
Note that parameter symbols can only be used for data values
&mdash; if you want to use dynamically determined table or column
names, you must insert them into the command string textually.
For example, if the preceding query needed to be done against a
dynamically selected table, you could do this:
<programlisting>
EXECUTE 'SELECT count(*) FROM '
|| tabname::regclass
|| ' WHERE inserted_by = $1 AND inserted <= $2'
INTO c
USING checked_user, checked_date;
</programlisting>
</para>
<para>
An <command>EXECUTE</> with a simple constant command string and some
<literal>USING</> parameters, as in the first example above, is
functionally equivalent to just writing the command directly in
<application>PL/pgSQL</application> and allowing replacement of
<application>PL/pgSQL</application> variables to happen automatically.
The important difference is that <command>EXECUTE</> will re-plan
the command on each execution, generating a plan that is specific
to the current parameter values; whereas
<application>PL/pgSQL</application> normally creates a generic plan
and caches it for re-use. In situations where the best plan depends
strongly on the parameter values, <command>EXECUTE</> can be
significantly faster; while when the plan is not sensitive to parameter
values, re-planning will be a waste.
</para>
<para>
<command>SELECT INTO</command> is not currently supported within
<command>EXECUTE</command>; instead, execute a plain <command>SELECT</>
command and specify <literal>INTO</> as part of the <command>EXECUTE</>
itself.
</para>
<note>
<para>
The <application>PL/pgSQL</application>
<command>EXECUTE</command> statement is not related to the
<xref linkend="sql-execute" endterm="sql-execute-title"> SQL
statement supported by the
<productname>PostgreSQL</productname> server. The server's
<command>EXECUTE</command> statement cannot be used directly within
<application>PL/pgSQL</> functions (and is not needed).
</para>
</note>
<example id="plpgsql-quote-literal-example">
<title>Quoting values in dynamic queries</title>
<indexterm>
<primary>quote_ident</primary>
<secondary>use in PL/PgSQL</secondary>
</indexterm>
<indexterm>
<primary>quote_literal</primary>
<secondary>use in PL/PgSQL</secondary>
</indexterm>
<indexterm>
<primary>quote_nullable</primary>
<secondary>use in PL/PgSQL</secondary>
</indexterm>
<para>
When working with dynamic commands you will often have to handle escaping
of single quotes. The recommended method for quoting fixed text in your
function body is dollar quoting. (If you have legacy code that does
not use dollar quoting, please refer to the
overview in <xref linkend="plpgsql-quote-tips">, which can save you
some effort when translating said code to a more reasonable scheme.)
</para>
<para>
Dynamic values that are to be inserted into the constructed
query require careful handling since they might themselves contain
quote characters.
An example (this assumes that you are using dollar quoting for the
function as a whole, so the quote marks need not be doubled):
<programlisting>
EXECUTE 'UPDATE tbl SET '
|| quote_ident(colname)
|| ' = '
|| quote_literal(newvalue)
|| ' WHERE key = '
|| quote_literal(keyvalue);
</programlisting>
</para>
<para>
This example demonstrates the use of the
<function>quote_ident</function> and
<function>quote_literal</function> functions (see <xref
linkend="functions-string">). For safety, expressions containing column
or table identifiers should be passed through
<function>quote_ident</function> before insertion in a dynamic query.
Expressions containing values that should be literal strings in the
constructed command should be passed through <function>quote_literal</>.
These functions take the appropriate steps to return the input text
enclosed in double or single quotes respectively, with any embedded
special characters properly escaped.
</para>
<para>
Because <function>quote_literal</function> is labelled
<literal>STRICT</literal>, it will always return null when called with a
null argument. In the above example, if <literal>newvalue</> or
<literal>keyvalue</> were null, the entire dynamic query string would
become null, leading to an error from <command>EXECUTE</command>.
You can avoid this problem by using the <function>quote_nullable</>
function, which works the same as <function>quote_literal</> except that
when called with a null argument it returns the string <literal>NULL</>.
For example,
<programlisting>
EXECUTE 'UPDATE tbl SET '
|| quote_ident(colname)
|| ' = '
|| quote_nullable(newvalue)
|| ' WHERE key = '
|| quote_nullable(keyvalue);
</programlisting>
If you are dealing with values that might be null, you should usually
use <function>quote_nullable</> in place of <function>quote_literal</>.
</para>
<para>
As always, care must be taken to ensure that null values in a query do
not deliver unintended results. For example the <literal>WHERE</> clause
<programlisting>
'WHERE key = ' || quote_nullable(keyvalue)
</programlisting>
will never succeed if <literal>keyvalue</> is null, because the
result of using the equality operator <literal>=</> with a null operand
is always null. If you wish null to work like an ordinary key value,
you would need to rewrite the above as
<programlisting>
'WHERE key IS NOT DISTINCT FROM ' || quote_nullable(keyvalue)
</programlisting>
(At present, <literal>IS NOT DISTINCT FROM</> is handled much less
efficiently than <literal>=</>, so don't do this unless you must.
See <xref linkend="functions-comparison"> for
more information on nulls and <literal>IS DISTINCT</>.)
</para>
<para>
Note that dollar quoting is only useful for quoting fixed text.
It would be a very bad idea to try to write this example as:
<programlisting>
EXECUTE 'UPDATE tbl SET '
|| quote_ident(colname)
|| ' = $$'
|| newvalue
|| '$$ WHERE key = '
|| quote_literal(keyvalue);
</programlisting>
because it would break if the contents of <literal>newvalue</>
happened to contain <literal>$$</>. The same objection would
apply to any other dollar-quoting delimiter you might pick.
So, to safely quote text that is not known in advance, you
<emphasis>must</> use <function>quote_literal</>,
<function>quote_nullable</>, or <function>quote_ident</>, as appropriate.
</para>
</example>
<para>
A much larger example of a dynamic command and
<command>EXECUTE</command> can be seen in <xref
linkend="plpgsql-porting-ex2">, which builds and executes a
<command>CREATE FUNCTION</> command to define a new function.
</para>
</sect2>
<sect2 id="plpgsql-statements-diagnostics">
<title>Obtaining the Result Status</title>
<para>
There are several ways to determine the effect of a command. The
first method is to use the <command>GET DIAGNOSTICS</command>
command, which has the form:
<synopsis>
GET DIAGNOSTICS <replaceable>variable</replaceable> = <replaceable>item</replaceable> <optional> , ... </optional>;
</synopsis>
This command allows retrieval of system status indicators. Each
<replaceable>item</replaceable> is a key word identifying a state
value to be assigned to the specified variable (which should be
of the right data type to receive it). The currently available
status items are <varname>ROW_COUNT</>, the number of rows
processed by the last <acronym>SQL</acronym> command sent down to
the <acronym>SQL</acronym> engine, and <varname>RESULT_OID</>,
the OID of the last row inserted by the most recent
<acronym>SQL</acronym> command. Note that <varname>RESULT_OID</>
is only useful after an <command>INSERT</command> command into a
table containing OIDs.
</para>
<para>
An example:
<programlisting>
GET DIAGNOSTICS integer_var = ROW_COUNT;
</programlisting>
</para>
<para>
The second method to determine the effects of a command is to check the
special variable named <literal>FOUND</literal>, which is of
type <type>boolean</type>. <literal>FOUND</literal> starts out
false within each <application>PL/pgSQL</application> function call.
It is set by each of the following types of statements:
<itemizedlist>
<listitem>
<para>
A <command>SELECT INTO</command> statement sets
<literal>FOUND</literal> true if a row is assigned, false if no
row is returned.
</para>
</listitem>
<listitem>
<para>
A <command>PERFORM</> statement sets <literal>FOUND</literal>
true if it produces (and discards) one or more rows, false if
no row is produced.
</para>
</listitem>
<listitem>
<para>
<command>UPDATE</>, <command>INSERT</>, and <command>DELETE</>
statements set <literal>FOUND</literal> true if at least one
row is affected, false if no row is affected.
</para>
</listitem>
<listitem>
<para>
A <command>FETCH</> statement sets <literal>FOUND</literal>
true if it returns a row, false if no row is returned.
</para>
</listitem>
<listitem>
<para>
A <command>MOVE</> statement sets <literal>FOUND</literal>
true if it successfully repositions the cursor, false otherwise.
</para>
</listitem>
<listitem>
<para>
A <command>FOR</> statement sets <literal>FOUND</literal> true
if it iterates one or more times, else false. This applies to
all four variants of the <command>FOR</> statement (integer
<command>FOR</> loops, record-set <command>FOR</> loops,
dynamic record-set <command>FOR</> loops, and cursor
<command>FOR</> loops).
<literal>FOUND</literal> is set this way when the
<command>FOR</> loop exits; inside the execution of the loop,
<literal>FOUND</literal> is not modified by the
<command>FOR</> statement, although it might be changed by the
execution of other statements within the loop body.
</para>
</listitem>
</itemizedlist>
<literal>FOUND</literal> is a local variable within each
<application>PL/pgSQL</application> function; any changes to it
affect only the current function.
</para>
</sect2>
<sect2 id="plpgsql-statements-null">
<title>Doing Nothing At All</title>
<para>
Sometimes a placeholder statement that does nothing is useful.
For example, it can indicate that one arm of an if/then/else
chain is deliberately empty. For this purpose, use the
<command>NULL</command> statement:
<synopsis>
NULL;
</synopsis>
</para>
<para>
For example, the following two fragments of code are equivalent:
<programlisting>
BEGIN
y := x / 0;
EXCEPTION
WHEN division_by_zero THEN
NULL; -- ignore the error
END;
</programlisting>
<programlisting>
BEGIN
y := x / 0;
EXCEPTION
WHEN division_by_zero THEN -- ignore the error
END;
</programlisting>
Which is preferable is a matter of taste.
</para>
<note>
<para>
In Oracle's PL/SQL, empty statement lists are not allowed, and so
<command>NULL</> statements are <emphasis>required</> for situations
such as this. <application>PL/pgSQL</application> allows you to
just write nothing, instead.
</para>
</note>
</sect2>
</sect1>
<sect1 id="plpgsql-control-structures">
<title>Control Structures</title>
<para>
Control structures are probably the most useful (and
important) part of <application>PL/pgSQL</>. With
<application>PL/pgSQL</>'s control structures,
you can manipulate <productname>PostgreSQL</> data in a very
flexible and powerful way.
</para>
<sect2 id="plpgsql-statements-returning">
<title>Returning From a Function</title>
<para>
There are two commands available that allow you to return data
from a function: <command>RETURN</command> and <command>RETURN
NEXT</command>.
</para>
<sect3>
<title><command>RETURN</></title>
<synopsis>
RETURN <replaceable>expression</replaceable>;
</synopsis>
<para>
<command>RETURN</command> with an expression terminates the
function and returns the value of
<replaceable>expression</replaceable> to the caller. This form
is to be used for <application>PL/pgSQL</> functions that do
not return a set.
</para>
<para>
When returning a scalar type, any expression can be used. The
expression's result will be automatically cast into the
function's return type as described for assignments. To return a
composite (row) value, you must write a record or row variable
as the <replaceable>expression</replaceable>.
</para>
<para>
If you declared the function with output parameters, write just
<command>RETURN</command> with no expression. The current values
of the output parameter variables will be returned.
</para>
<para>
If you declared the function to return <type>void</type>, a
<command>RETURN</command> statement can be used to exit the function
early; but do not write an expression following
<command>RETURN</command>.
</para>
<para>
The return value of a function cannot be left undefined. If
control reaches the end of the top-level block of the function
without hitting a <command>RETURN</command> statement, a run-time
error will occur. This restriction does not apply to functions
with output parameters and functions returning <type>void</type>,
however. In those cases a <command>RETURN</command> statement is
automatically executed if the top-level block finishes.
</para>
</sect3>
<sect3>
<title><command>RETURN NEXT</> and <command>RETURN QUERY</command></title>
<indexterm>
<primary>RETURN NEXT</primary>
<secondary>in PL/PgSQL</secondary>
</indexterm>
<indexterm>
<primary>RETURN QUERY</primary>
<secondary>in PL/PgSQL</secondary>
</indexterm>
<synopsis>
RETURN NEXT <replaceable>expression</replaceable>;
RETURN QUERY <replaceable>query</replaceable>;
RETURN QUERY EXECUTE <replaceable class="command">command-string</replaceable> <optional> USING <replaceable>expression</replaceable> <optional>, ...</optional> </optional>;
</synopsis>
<para>
When a <application>PL/pgSQL</> function is declared to return
<literal>SETOF <replaceable>sometype</></literal>, the procedure
to follow is slightly different. In that case, the individual
items to return are specified by a sequence of <command>RETURN
NEXT</command> or <command>RETURN QUERY</command> commands, and
then a final <command>RETURN</command> command with no argument
is used to indicate that the function has finished executing.
<command>RETURN NEXT</command> can be used with both scalar and
composite data types; with a composite result type, an entire
<quote>table</quote> of results will be returned.
<command>RETURN QUERY</command> appends the results of executing
a query to the function's result set. <command>RETURN
NEXT</command> and <command>RETURN QUERY</command> can be freely
intermixed in a single set-returning function, in which case
their results will be concatenated.
</para>
<para>
<command>RETURN NEXT</command> and <command>RETURN
QUERY</command> do not actually return from the function &mdash;
they simply append zero or more rows to the function's result
set. Execution then continues with the next statement in the
<application>PL/pgSQL</> function. As successive
<command>RETURN NEXT</command> or <command>RETURN
QUERY</command> commands are executed, the result set is built
up. A final <command>RETURN</command>, which should have no
argument, causes control to exit the function (or you can just
let control reach the end of the function).
</para>
<para>
<command>RETURN QUERY</command> has a variant
<command>RETURN QUERY EXECUTE</command>, which specifies the
query to be executed dynamically. Parameter expressions can
be inserted into the computed query string via <literal>USING</>,
in just the same way as in the <command>EXECUTE</> command.
</para>
<para>
If you declared the function with output parameters, write just
<command>RETURN NEXT</command> with no expression. On each
execution, the current values of the output parameter
variable(s) will be saved for eventual return as a row of the
result. Note that you must declare the function as returning
<literal>SETOF record</literal> when there are multiple output
parameters, or <literal>SETOF <replaceable>sometype</></literal>
when there is just one output parameter of type
<replaceable>sometype</>, in order to create a set-returning
function with output parameters.
</para>
<para>
Here is an example of a function using <command>RETURN
NEXT</command>:
<programlisting>
CREATE TABLE foo (fooid INT, foosubid INT, fooname TEXT);
INSERT INTO foo VALUES (1, 2, 'three');
INSERT INTO foo VALUES (4, 5, 'six');
CREATE OR REPLACE FUNCTION getAllFoo() RETURNS SETOF foo AS
$BODY$
DECLARE
r foo%rowtype;
BEGIN
FOR r IN SELECT * FROM foo
WHERE fooid &gt; 0
LOOP
-- can do some processing here
RETURN NEXT r; -- return current row of SELECT
END LOOP;
RETURN;
END
$BODY$
LANGUAGE 'plpgsql' ;
SELECT * FROM getallfoo();
</programlisting>
</para>
<note>
<para>
The current implementation of <command>RETURN NEXT</command>
and <command>RETURN QUERY</command> stores the entire result set
before returning from the function, as discussed above. That
means that if a <application>PL/pgSQL</> function produces a
very large result set, performance might be poor: data will be
written to disk to avoid memory exhaustion, but the function
itself will not return until the entire result set has been
generated. A future version of <application>PL/pgSQL</> might
allow users to define set-returning functions
that do not have this limitation. Currently, the point at
which data begins being written to disk is controlled by the
<xref linkend="guc-work-mem">
configuration variable. Administrators who have sufficient
memory to store larger result sets in memory should consider
increasing this parameter.
</para>
</note>
</sect3>
</sect2>
<sect2 id="plpgsql-conditionals">
<title>Conditionals</title>
<para>
<command>IF</> and <command>CASE</> statements let you execute
alternative commands based on certain conditions.
<application>PL/pgSQL</> has three forms of <command>IF</>:
<itemizedlist>
<listitem>
<para><literal>IF ... THEN</></>
</listitem>
<listitem>
<para><literal>IF ... THEN ... ELSE</></>
</listitem>
<listitem>
<para><literal>IF ... THEN ... ELSIF ... THEN ... ELSE</></>
</listitem>
</itemizedlist>
and two forms of <command>CASE</>:
<itemizedlist>
<listitem>
<para><literal>CASE ... WHEN ... THEN ... ELSE ... END CASE</></>
</listitem>
<listitem>
<para><literal>CASE WHEN ... THEN ... ELSE ... END CASE</></>
</listitem>
</itemizedlist>
</para>
<sect3>
<title><literal>IF-THEN</></title>
<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
END IF;
</synopsis>
<para>
<literal>IF-THEN</literal> statements are the simplest form of
<literal>IF</literal>. The statements between
<literal>THEN</literal> and <literal>END IF</literal> will be
executed if the condition is true. Otherwise, they are
skipped.
</para>
<para>
Example:
<programlisting>
IF v_user_id &lt;&gt; 0 THEN
UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>IF-THEN-ELSE</></title>
<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
ELSE
<replaceable>statements</replaceable>
END IF;
</synopsis>
<para>
<literal>IF-THEN-ELSE</literal> statements add to
<literal>IF-THEN</literal> by letting you specify an
alternative set of statements that should be executed if the
condition is not true. (Note this includes the case where the
condition evaluates to NULL.)
</para>
<para>
Examples:
<programlisting>
IF parentid IS NULL OR parentid = ''
THEN
RETURN fullname;
ELSE
RETURN hp_true_filename(parentid) || '/' || fullname;
END IF;
</programlisting>
<programlisting>
IF v_count &gt; 0 THEN
INSERT INTO users_count (count) VALUES (v_count);
RETURN 't';
ELSE
RETURN 'f';
END IF;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>IF-THEN-ELSIF</></title>
<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
...
</optional>
</optional>
<optional> ELSE
<replaceable>statements</replaceable> </optional>
END IF;
</synopsis>
<para>
Sometimes there are more than just two alternatives.
<literal>IF-THEN-ELSIF</> provides a convenient
method of checking several alternatives in turn.
The <literal>IF</> conditions are tested successively
until the first one that is true is found. Then the
associated statement(s) are executed, after which control
passes to the next statement after <literal>END IF</>.
(Any subsequent <literal>IF</> conditions are <emphasis>not</>
tested.) If none of the <literal>IF</> conditions is true,
then the <literal>ELSE</> block (if any) is executed.
</para>
<para>
Here is an example:
<programlisting>
IF number = 0 THEN
result := 'zero';
ELSIF number &gt; 0 THEN
result := 'positive';
ELSIF number &lt; 0 THEN
result := 'negative';
ELSE
-- hmm, the only other possibility is that number is null
result := 'NULL';
END IF;
</programlisting>
</para>
<para>
The key word <literal>ELSIF</> can also be spelled
<literal>ELSEIF</>.
</para>
<para>
An alternative way of accomplishing the same task is to nest
<literal>IF-THEN-ELSE</literal> statements, as in the
following example:
<programlisting>
IF demo_row.sex = 'm' THEN
pretty_sex := 'man';
ELSE
IF demo_row.sex = 'f' THEN
pretty_sex := 'woman';
END IF;
END IF;
</programlisting>
</para>
<para>
However, this method requires writing a matching <literal>END IF</>
for each <literal>IF</>, so it is much more cumbersome than
using <literal>ELSIF</> when there are many alternatives.
</para>
</sect3>
<sect3>
<title>Simple <literal>CASE</></title>
<synopsis>
CASE <replaceable>search-expression</replaceable>
WHEN <replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> <optional> ... </optional></optional> THEN
<replaceable>statements</replaceable>
<optional> WHEN <replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> <optional> ... </optional></optional> THEN
<replaceable>statements</replaceable>
... </optional>
<optional> ELSE
<replaceable>statements</replaceable> </optional>
END CASE;
</synopsis>
<para>
The simple form of <command>CASE</> provides conditional execution
based on equality of operands. The <replaceable>search-expression</>
is evaluated (once) and successively compared to each
<replaceable>expression</> in the <literal>WHEN</> clauses.
If a match is found, then the corresponding
<replaceable>statements</replaceable> are executed, and then control
passes to the next statement after <literal>END CASE</>. (Subsequent
<literal>WHEN</> expressions are not evaluated.) If no match is
found, the <literal>ELSE</> <replaceable>statements</replaceable> are
executed; but if <literal>ELSE</> is not present, then a
<literal>CASE_NOT_FOUND</literal> exception is raised.
</para>
<para>
Here is a simple example:
<programlisting>
CASE x
WHEN 1, 2 THEN
msg := 'one or two';
ELSE
msg := 'other value than one or two';
END CASE;
</programlisting>
</para>
</sect3>
<sect3>
<title>Searched <literal>CASE</></title>
<synopsis>
CASE
WHEN <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
<optional> WHEN <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
... </optional>
<optional> ELSE
<replaceable>statements</replaceable> </optional>
END CASE;
</synopsis>
<para>
The searched form of <command>CASE</> provides conditional execution
based on truth of boolean expressions. Each <literal>WHEN</> clause's
<replaceable>boolean-expression</replaceable> is evaluated in turn,
until one is found that yields <literal>true</>. Then the
corresponding <replaceable>statements</replaceable> are executed, and
then control passes to the next statement after <literal>END CASE</>.
(Subsequent <literal>WHEN</> expressions are not evaluated.)
If no true result is found, the <literal>ELSE</>
<replaceable>statements</replaceable> are executed;
but if <literal>ELSE</> is not present, then a
<literal>CASE_NOT_FOUND</literal> exception is raised.
</para>
<para>
Here is an example:
<programlisting>
CASE
WHEN x BETWEEN 0 AND 10 THEN
msg := 'value is between zero and ten';
WHEN x BETWEEN 11 AND 20 THEN
msg := 'value is between eleven and twenty';
END CASE;
</programlisting>
</para>
<para>
This form of <command>CASE</> is entirely equivalent to
<literal>IF-THEN-ELSIF</>, except for the rule that reaching
an omitted <literal>ELSE</> clause results in an error rather
than doing nothing.
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-control-structures-loops">
<title>Simple Loops</title>
<indexterm zone="plpgsql-control-structures-loops">
<primary>loop</primary>
<secondary>in PL/pgSQL</secondary>
</indexterm>
<para>
With the <literal>LOOP</>, <literal>EXIT</>,
<literal>CONTINUE</>, <literal>WHILE</>, and <literal>FOR</>
statements, you can arrange for your <application>PL/pgSQL</>
function to repeat a series of commands.
</para>
<sect3>
<title><literal>LOOP</></title>
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
LOOP
<replaceable>statements</replaceable>
END LOOP <optional> <replaceable>label</replaceable> </optional>;
</synopsis>
<para>
<literal>LOOP</> defines an unconditional loop that is repeated
indefinitely until terminated by an <literal>EXIT</> or
<command>RETURN</command> statement. The optional
<replaceable>label</replaceable> can be used by <literal>EXIT</>
and <literal>CONTINUE</literal> statements in nested loops to
specify which loop the statement should be applied to.
</para>
</sect3>
<sect3>
<title><literal>EXIT</></title>
<indexterm>
<primary>EXIT</primary>
<secondary>in PL/pgSQL</secondary>
</indexterm>
<synopsis>
EXIT <optional> <replaceable>label</replaceable> </optional> <optional> WHEN <replaceable>boolean-expression</replaceable> </optional>;
</synopsis>
<para>
If no <replaceable>label</replaceable> is given, the innermost
loop is terminated and the statement following <literal>END
LOOP</> is executed next. If <replaceable>label</replaceable>
is given, it must be the label of the current or some outer
level of nested loop or block. Then the named loop or block is
terminated and control continues with the statement after the
loop's/block's corresponding <literal>END</>.
</para>
<para>
If <literal>WHEN</> is specified, the loop exit occurs only if
<replaceable>boolean-expression</> is true. Otherwise, control passes
to the statement after <literal>EXIT</>.
</para>
<para>
<literal>EXIT</> can be used with all types of loops; it is
not limited to use with unconditional loops. When used with a
<literal>BEGIN</literal> block, <literal>EXIT</literal> passes
control to the next statement after the end of the block.
</para>
<para>
Examples:
<programlisting>
LOOP
-- some computations
IF count &gt; 0 THEN
EXIT; -- exit loop
END IF;
END LOOP;
LOOP
-- some computations
EXIT WHEN count &gt; 0; -- same result as previous example
END LOOP;
BEGIN
-- some computations
IF stocks &gt; 100000 THEN
EXIT; -- causes exit from the BEGIN block
END IF;
END;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>CONTINUE</></title>
<indexterm>
<primary>CONTINUE</primary>
<secondary>in PL/pgSQL</secondary>
</indexterm>
<synopsis>
CONTINUE <optional> <replaceable>label</replaceable> </optional> <optional> WHEN <replaceable>boolean-expression</replaceable> </optional>;
</synopsis>
<para>
If no <replaceable>label</> is given, the next iteration of
the innermost loop is begun. That is, all statements remaining
in the loop body are skipped, and control returns
to the loop control expression (if any) to determine whether
another loop iteration is needed.
If <replaceable>label</> is present, it
specifies the label of the loop whose execution will be
continued.
</para>
<para>
If <literal>WHEN</> is specified, the next iteration of the
loop is begun only if <replaceable>boolean-expression</> is
true. Otherwise, control passes to the statement after
<literal>CONTINUE</>.
</para>
<para>
<literal>CONTINUE</> can be used with all types of loops; it
is not limited to use with unconditional loops.
</para>
<para>
Examples:
<programlisting>
LOOP
-- some computations
EXIT WHEN count &gt; 100;
CONTINUE WHEN count &lt; 50;
-- some computations for count IN [50 .. 100]
END LOOP;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>WHILE</></title>
<indexterm>
<primary>WHILE</primary>
<secondary>in PL/pgSQL</secondary>
</indexterm>
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
WHILE <replaceable>boolean-expression</replaceable> LOOP
<replaceable>statements</replaceable>
END LOOP <optional> <replaceable>label</replaceable> </optional>;
</synopsis>
<para>
The <literal>WHILE</> statement repeats a
sequence of statements so long as the
<replaceable>boolean-expression</replaceable>
evaluates to true. The expression is checked just before
each entry to the loop body.
</para>
<para>
For example:
<programlisting>
WHILE amount_owed &gt; 0 AND gift_certificate_balance &gt; 0 LOOP
-- some computations here
END LOOP;
WHILE NOT done LOOP
-- some computations here
END LOOP;
</programlisting>
</para>
</sect3>
<sect3 id="plpgsql-integer-for">
<title><literal>FOR</> (integer variant)</title>
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
FOR <replaceable>name</replaceable> IN <optional> REVERSE </optional> <replaceable>expression</replaceable> .. <replaceable>expression</replaceable> <optional> BY <replaceable>expression</replaceable> </optional> LOOP
<replaceable>statements</replaceable>
END LOOP <optional> <replaceable>label</replaceable> </optional>;
</synopsis>
<para>
This form of <literal>FOR</> creates a loop that iterates over a range
of integer values. The variable
<replaceable>name</replaceable> is automatically defined as type
<type>integer</> and exists only inside the loop (any existing
definition of the variable name is ignored within the loop).
The two expressions giving
the lower and upper bound of the range are evaluated once when entering
the loop. If the <literal>BY</> clause isn't specified the iteration
step is 1, otherwise it's the value specified in the <literal>BY</>
clause, which again is evaluated once on loop entry.
If <literal>REVERSE</> is specified then the step value is
subtracted, rather than added, after each iteration.
</para>
<para>
Some examples of integer <literal>FOR</> loops:
<programlisting>
FOR i IN 1..10 LOOP
-- i will take on the values 1,2,3,4,5,6,7,8,9,10 within the loop
END LOOP;
FOR i IN REVERSE 10..1 LOOP
-- i will take on the values 10,9,8,7,6,5,4,3,2,1 within the loop
END LOOP;
FOR i IN REVERSE 10..1 BY 2 LOOP
-- i will take on the values 10,8,6,4,2 within the loop
END LOOP;
</programlisting>
</para>
<para>
If the lower bound is greater than the upper bound (or less than,
in the <literal>REVERSE</> case), the loop body is not
executed at all. No error is raised.
</para>
<para>
If a <replaceable>label</replaceable> is attached to the
<literal>FOR</> loop then the integer loop variable can be
referenced with a qualified name, using that
<replaceable>label</replaceable>.
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-records-iterating">
<title>Looping Through Query Results</title>
<para>
Using a different type of <literal>FOR</> loop, you can iterate through
the results of a query and manipulate that data
accordingly. The syntax is:
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
FOR <replaceable>target</replaceable> IN <replaceable>query</replaceable> LOOP
<replaceable>statements</replaceable>
END LOOP <optional> <replaceable>label</replaceable> </optional>;
</synopsis>
The <replaceable>target</replaceable> is a record variable, row variable,
or comma-separated list of scalar variables.
The <replaceable>target</replaceable> is successively assigned each row
resulting from the <replaceable>query</replaceable> and the loop body is
executed for each row. Here is an example:
<programlisting>
CREATE FUNCTION cs_refresh_mviews() RETURNS integer AS $$
DECLARE
mviews RECORD;
BEGIN
PERFORM cs_log('Refreshing materialized views...');
FOR mviews IN SELECT * FROM cs_materialized_views ORDER BY sort_key LOOP
-- Now "mviews" has one record from cs_materialized_views
PERFORM cs_log('Refreshing materialized view ' || quote_ident(mviews.mv_name) || ' ...');
EXECUTE 'TRUNCATE TABLE ' || quote_ident(mviews.mv_name);
EXECUTE 'INSERT INTO ' || quote_ident(mviews.mv_name) || ' ' || mviews.mv_query;
END LOOP;
PERFORM cs_log('Done refreshing materialized views.');
RETURN 1;
END;
$$ LANGUAGE plpgsql;
</programlisting>
If the loop is terminated by an <literal>EXIT</> statement, the last
assigned row value is still accessible after the loop.
</para>
<para>
The <replaceable>query</replaceable> used in this type of <literal>FOR</>
statement can be any SQL command that returns rows to the caller:
<command>SELECT</> is the most common case,
but you can also use <command>INSERT</>, <command>UPDATE</>, or
<command>DELETE</> with a <literal>RETURNING</> clause. Some utility
commands such as <command>EXPLAIN</> will work too.
</para>
<para>
<application>PL/pgSQL</> variables are substituted into the query text,
and the query plan is cached for possible re-use, as discussed in
detail in <xref linkend="plpgsql-var-subst"> and
<xref linkend="plpgsql-plan-caching">.
</para>
<para>
The <literal>FOR-IN-EXECUTE</> statement is another way to iterate over
rows:
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
FOR <replaceable>target</replaceable> IN EXECUTE <replaceable>text_expression</replaceable> <optional> USING <replaceable>expression</replaceable> <optional>, ...</optional> </optional> LOOP
<replaceable>statements</replaceable>
END LOOP <optional> <replaceable>label</replaceable> </optional>;
</synopsis>
This is like the previous form, except that the source query
is specified as a string expression, which is evaluated and replanned
on each entry to the <literal>FOR</> loop. This allows the programmer to
choose the speed of a preplanned query or the flexibility of a dynamic
query, just as with a plain <command>EXECUTE</command> statement.
As with <command>EXECUTE</command>, parameter values can be inserted
into the dynamic command via <literal>USING</>.
</para>
<para>
Another way to specify the query whose results should be iterated
through is to declare it as a cursor. This is described in
<xref linkend="plpgsql-cursor-for-loop">.
</para>
</sect2>
<sect2 id="plpgsql-error-trapping">
<title>Trapping Errors</title>
<indexterm>
<primary>exceptions</primary>
<secondary>in PL/PgSQL</secondary>
</indexterm>
<para>
By default, any error occurring in a <application>PL/pgSQL</>
function aborts execution of the function, and indeed of the
surrounding transaction as well. You can trap errors and recover
from them by using a <command>BEGIN</> block with an
<literal>EXCEPTION</> clause. The syntax is an extension of the
normal syntax for a <command>BEGIN</> block:
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
<optional> DECLARE
<replaceable>declarations</replaceable> </optional>
BEGIN
<replaceable>statements</replaceable>
EXCEPTION
WHEN <replaceable>condition</replaceable> <optional> OR <replaceable>condition</replaceable> ... </optional> THEN
<replaceable>handler_statements</replaceable>
<optional> WHEN <replaceable>condition</replaceable> <optional> OR <replaceable>condition</replaceable> ... </optional> THEN
<replaceable>handler_statements</replaceable>
... </optional>
END;
</synopsis>
</para>
<para>
If no error occurs, this form of block simply executes all the
<replaceable>statements</replaceable>, and then control passes
to the next statement after <literal>END</>. But if an error
occurs within the <replaceable>statements</replaceable>, further
processing of the <replaceable>statements</replaceable> is
abandoned, and control passes to the <literal>EXCEPTION</> list.
The list is searched for the first <replaceable>condition</replaceable>
matching the error that occurred. If a match is found, the
corresponding <replaceable>handler_statements</replaceable> are
executed, and then control passes to the next statement after
<literal>END</>. If no match is found, the error propagates out
as though the <literal>EXCEPTION</> clause were not there at all:
the error can be caught by an enclosing block with
<literal>EXCEPTION</>, or if there is none it aborts processing
of the function.
</para>
<para>
The <replaceable>condition</replaceable> names can be any of
those shown in <xref linkend="errcodes-appendix">. A category
name matches any error within its category. The special
condition name <literal>OTHERS</> matches every error type except
<literal>QUERY_CANCELED</>. (It is possible, but often unwise,
to trap <literal>QUERY_CANCELED</> by name.) Condition names are
not case-sensitive. Also, an error condition can be specified
by <literal>SQLSTATE</> code; for example these are equivalent:
<programlisting>
WHEN division_by_zero THEN ...
WHEN SQLSTATE '22012' THEN ...
</programlisting>
</para>
<para>
If a new error occurs within the selected
<replaceable>handler_statements</replaceable>, it cannot be caught
by this <literal>EXCEPTION</> clause, but is propagated out.
A surrounding <literal>EXCEPTION</> clause could catch it.
</para>
<para>
When an error is caught by an <literal>EXCEPTION</> clause,
the local variables of the <application>PL/pgSQL</> function
remain as they were when the error occurred, but all changes
to persistent database state within the block are rolled back.
As an example, consider this fragment:
<programlisting>
INSERT INTO mytab(firstname, lastname) VALUES('Tom', 'Jones');
BEGIN
UPDATE mytab SET firstname = 'Joe' WHERE lastname = 'Jones';
x := x + 1;
y := x / 0;
EXCEPTION
WHEN division_by_zero THEN
RAISE NOTICE 'caught division_by_zero';
RETURN x;
END;
</programlisting>
When control reaches the assignment to <literal>y</>, it will
fail with a <literal>division_by_zero</> error. This will be caught by
the <literal>EXCEPTION</> clause. The value returned in the
<command>RETURN</> statement will be the incremented value of
<literal>x</>, but the effects of the <command>UPDATE</> command will
have been rolled back. The <command>INSERT</> command preceding the
block is not rolled back, however, so the end result is that the database
contains <literal>Tom Jones</> not <literal>Joe Jones</>.
</para>
<tip>
<para>
A block containing an <literal>EXCEPTION</> clause is significantly
more expensive to enter and exit than a block without one. Therefore,
don't use <literal>EXCEPTION</> without need.
</para>
</tip>
<para>
Within an exception handler, the <varname>SQLSTATE</varname>
variable contains the error code that corresponds to the
exception that was raised (refer to <xref
linkend="errcodes-table"> for a list of possible error
codes). The <varname>SQLERRM</varname> variable contains the
error message associated with the exception. These variables are
undefined outside exception handlers.
</para>
<example id="plpgsql-upsert-example">
<title>Exceptions with <command>UPDATE</>/<command>INSERT</></title>
<para>
This example uses exception handling to perform either
<command>UPDATE</> or <command>INSERT</>, as appropriate:
<programlisting>
CREATE TABLE db (a INT PRIMARY KEY, b TEXT);
CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
LOOP
-- first try to update the key
UPDATE db SET b = data WHERE a = key;
IF found THEN
RETURN;
END IF;
-- not there, so try to insert the key
-- if someone else inserts the same key concurrently,
-- we could get a unique-key failure
BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
-- do nothing, and loop to try the UPDATE again
END;
END LOOP;
END;
$$
LANGUAGE plpgsql;
SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');
</programlisting>
</para>
</example>
</sect2>
</sect1>
<sect1 id="plpgsql-cursors">
<title>Cursors</title>
<indexterm zone="plpgsql-cursors">
<primary>cursor</primary>
<secondary>in PL/pgSQL</secondary>
</indexterm>
<para>
Rather than executing a whole query at once, it is possible to set
up a <firstterm>cursor</> that encapsulates the query, and then read
the query result a few rows at a time. One reason for doing this is
to avoid memory overrun when the result contains a large number of
rows. (However, <application>PL/pgSQL</> users do not normally need
to worry about that, since <literal>FOR</> loops automatically use a cursor
internally to avoid memory problems.) A more interesting usage is to
return a reference to a cursor that a function has created, allowing the
caller to read the rows. This provides an efficient way to return
large row sets from functions.
</para>
<sect2 id="plpgsql-cursor-declarations">
<title>Declaring Cursor Variables</title>
<para>
All access to cursors in <application>PL/pgSQL</> goes through
cursor variables, which are always of the special data type
<type>refcursor</>. One way to create a cursor variable
is just to declare it as a variable of type <type>refcursor</>.
Another way is to use the cursor declaration syntax,
which in general is:
<synopsis>
<replaceable>name</replaceable> <optional> <optional> NO </optional> SCROLL </optional> CURSOR <optional> ( <replaceable>arguments</replaceable> ) </optional> FOR <replaceable>query</replaceable>;
</synopsis>
(<literal>FOR</> can be replaced by <literal>IS</> for
<productname>Oracle</productname> compatibility.)
If <literal>SCROLL</> is specified, the cursor will be capable of
scrolling backward; if <literal>NO SCROLL</> is specified, backward
fetches will be rejected; if neither specification appears, it is
query-dependent whether backward fetches will be allowed.
<replaceable>arguments</replaceable>, if specified, is a
comma-separated list of pairs <literal><replaceable>name</replaceable>
<replaceable>datatype</replaceable></literal> that define names to be
replaced by parameter values in the given query. The actual
values to substitute for these names will be specified later,
when the cursor is opened.
</para>
<para>
Some examples:
<programlisting>
DECLARE
curs1 refcursor;
curs2 CURSOR FOR SELECT * FROM tenk1;
curs3 CURSOR (key integer) IS SELECT * FROM tenk1 WHERE unique1 = key;
</programlisting>
All three of these variables have the data type <type>refcursor</>,
but the first can be used with any query, while the second has
a fully specified query already <firstterm>bound</> to it, and the last
has a parameterized query bound to it. (<literal>key</> will be
replaced by an integer parameter value when the cursor is opened.)
The variable <literal>curs1</>
is said to be <firstterm>unbound</> since it is not bound to
any particular query.
</para>
</sect2>
<sect2 id="plpgsql-cursor-opening">
<title>Opening Cursors</title>
<para>
Before a cursor can be used to retrieve rows, it must be
<firstterm>opened</>. (This is the equivalent action to the SQL
command <command>DECLARE CURSOR</>.) <application>PL/pgSQL</> has
three forms of the <command>OPEN</> statement, two of which use unbound
cursor variables while the third uses a bound cursor variable.
</para>
<note>
<para>
Bound cursors can also be used without explicitly opening them,
via the <command>FOR</> statement described in
<xref linkend="plpgsql-cursor-for-loop">.
</para>
</note>
<sect3>
<title><command>OPEN FOR</command> <replaceable>query</replaceable></title>
<synopsis>
OPEN <replaceable>unbound_cursor</replaceable> <optional> <optional> NO </optional> SCROLL </optional> FOR <replaceable>query</replaceable>;
</synopsis>
<para>
The cursor variable is opened and given the specified query to
execute. The cursor cannot be open already, and it must have been
declared as an unbound cursor (that is, as a simple
<type>refcursor</> variable). The query must be a
<command>SELECT</command>, or something else that returns rows
(such as <command>EXPLAIN</>). The query
is treated in the same way as other SQL commands in
<application>PL/pgSQL</>: <application>PL/pgSQL</>
variable names are substituted, and the query plan is cached for
possible reuse. When a <application>PL/pgSQL</>
variable is substituted into the cursor query, the value that is
substituted is the one it has at the time of the <command>OPEN</>;
subsequent changes to the variable will not affect the cursor's
behavior.
The <literal>SCROLL</> and <literal>NO SCROLL</>
options have the same meanings as for a bound cursor.
</para>
<para>
An example:
<programlisting>
OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;
</programlisting>
</para>
</sect3>
<sect3>
<title><command>OPEN FOR EXECUTE</command></title>
<synopsis>
OPEN <replaceable>unbound_cursor</replaceable> <optional> <optional> NO </optional> SCROLL </optional> FOR EXECUTE <replaceable class="command">query_string</replaceable>;
</synopsis>
<para>
The cursor variable is opened and given the specified query to
execute. The cursor cannot be open already, and it must have been
declared as an unbound cursor (that is, as a simple
<type>refcursor</> variable). The query is specified as a string
expression, in the same way as in the <command>EXECUTE</command>
command. As usual, this gives flexibility so the query plan can vary
from one run to the next (see <xref linkend="plpgsql-plan-caching">),
and it also means that variable substitution is not done on the
command string.
The <literal>SCROLL</> and
<literal>NO SCROLL</> options have the same meanings as for a bound
cursor.
</para>
<para>
An example:
<programlisting>
OPEN curs1 FOR EXECUTE 'SELECT * FROM ' || quote_ident($1);
</programlisting>
</para>
</sect3>
<sect3>
<title>Opening a Bound Cursor</title>
<synopsis>
OPEN <replaceable>bound_cursor</replaceable> <optional> ( <replaceable>argument_values</replaceable> ) </optional>;
</synopsis>
<para>
This form of <command>OPEN</command> is used to open a cursor
variable whose query was bound to it when it was declared. The
cursor cannot be open already. A list of actual argument value
expressions must appear if and only if the cursor was declared to
take arguments. These values will be substituted in the query.
The query plan for a bound cursor is always considered cacheable;
there is no equivalent of <command>EXECUTE</command> in this case.
Notice that <literal>SCROLL</> and
<literal>NO SCROLL</> cannot be specified, as the cursor's scrolling
behavior was already determined.
</para>
<para>
Note that because variable substitution is done on the bound
cursor's query, there are two ways to pass values into the cursor:
either with an explicit argument to <command>OPEN</>, or
implicitly by referencing a <application>PL/pgSQL</> variable
in the query. However, only variables declared before the bound
cursor was declared will be substituted into it. In either case
the value to be passed is determined at the time of the
<command>OPEN</>.
</para>
<para>
Examples:
<programlisting>
OPEN curs2;
OPEN curs3(42);
</programlisting>
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-cursor-using">
<title>Using Cursors</title>
<para>
Once a cursor has been opened, it can be manipulated with the
statements described here.
</para>
<para>
These manipulations need not occur in the same function that
opened the cursor to begin with. You can return a <type>refcursor</>
value out of a function and let the caller operate on the cursor.
(Internally, a <type>refcursor</> value is simply the string name
of a so-called portal containing the active query for the cursor. This name
can be passed around, assigned to other <type>refcursor</> variables,
and so on, without disturbing the portal.)
</para>
<para>
All portals are implicitly closed at transaction end. Therefore
a <type>refcursor</> value is usable to reference an open cursor
only until the end of the transaction.
</para>
<sect3>
<title><literal>FETCH</></title>
<synopsis>
FETCH <optional> <replaceable>direction</replaceable> { FROM | IN } </optional> <replaceable>cursor</replaceable> INTO <replaceable>target</replaceable>;
</synopsis>
<para>
<command>FETCH</command> retrieves the next row from the
cursor into a target, which might be a row variable, a record
variable, or a comma-separated list of simple variables, just like
<command>SELECT INTO</command>. If there is no next row, the
target is set to NULL(s). As with <command>SELECT
INTO</command>, the special variable <literal>FOUND</literal> can
be checked to see whether a row was obtained or not.
</para>
<para>
The <replaceable>direction</replaceable> clause can be any of the
variants allowed in the SQL <xref linkend="sql-fetch"
endterm="sql-fetch-title"> command except the ones that can fetch
more than one row; namely, it can be
<literal>NEXT</>,
<literal>PRIOR</>,
<literal>FIRST</>,
<literal>LAST</>,
<literal>ABSOLUTE</> <replaceable>count</replaceable>,
<literal>RELATIVE</> <replaceable>count</replaceable>,
<literal>FORWARD</>, or
<literal>BACKWARD</>.
Omitting <replaceable>direction</replaceable> is the same
as specifying <literal>NEXT</>.
<replaceable>direction</replaceable> values that require moving
backward are likely to fail unless the cursor was declared or opened
with the <literal>SCROLL</> option.
</para>
<para>
<replaceable>cursor</replaceable> must be the name of a <type>refcursor</>
variable that references an open cursor portal.
</para>
<para>
Examples:
<programlisting>
FETCH curs1 INTO rowvar;
FETCH curs2 INTO foo, bar, baz;
FETCH LAST FROM curs3 INTO x, y;
FETCH RELATIVE -2 FROM curs4 INTO x;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>MOVE</></title>
<synopsis>
MOVE <optional> <replaceable>direction</replaceable> { FROM | IN } </optional> <replaceable>cursor</replaceable>;
</synopsis>
<para>
<command>MOVE</command> repositions a cursor without retrieving
any data. <command>MOVE</command> works exactly like the
<command>FETCH</command> command, except it only repositions the
cursor and does not return the row moved to. As with <command>SELECT
INTO</command>, the special variable <literal>FOUND</literal> can
be checked to see whether there was a next row to move to.
</para>
<para>
The options for the <replaceable>direction</replaceable> clause are
the same as for <command>FETCH</>, namely
<literal>NEXT</>,
<literal>PRIOR</>,
<literal>FIRST</>,
<literal>LAST</>,
<literal>ABSOLUTE</> <replaceable>count</replaceable>,
<literal>RELATIVE</> <replaceable>count</replaceable>,
<literal>FORWARD</>, or
<literal>BACKWARD</>.
Omitting <replaceable>direction</replaceable> is the same
as specifying <literal>NEXT</>.
<replaceable>direction</replaceable> values that require moving
backward are likely to fail unless the cursor was declared or opened
with the <literal>SCROLL</> option.
</para>
<para>
Examples:
<programlisting>
MOVE curs1;
MOVE LAST FROM curs3;
MOVE RELATIVE -2 FROM curs4;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>UPDATE/DELETE WHERE CURRENT OF</></title>
<synopsis>
UPDATE <replaceable>table</replaceable> SET ... WHERE CURRENT OF <replaceable>cursor</replaceable>;
DELETE FROM <replaceable>table</replaceable> WHERE CURRENT OF <replaceable>cursor</replaceable>;
</synopsis>
<para>
When a cursor is positioned on a table row, that row can be updated
or deleted using the cursor to identify the row. Note that this
only works for simple (non-join, non-grouping) cursor queries.
For additional information see the
<xref linkend="sql-declare" endterm="sql-declare-title">
reference page.
</para>
<para>
An example:
<programlisting>
UPDATE foo SET dataval = myval WHERE CURRENT OF curs1;
</programlisting>
</para>
</sect3>
<sect3>
<title><literal>CLOSE</></title>
<synopsis>
CLOSE <replaceable>cursor</replaceable>;
</synopsis>
<para>
<command>CLOSE</command> closes the portal underlying an open
cursor. This can be used to release resources earlier than end of
transaction, or to free up the cursor variable to be opened again.
</para>
<para>
An example:
<programlisting>
CLOSE curs1;
</programlisting>
</para>
</sect3>
<sect3>
<title>Returning Cursors</title>
<para>
<application>PL/pgSQL</> functions can return cursors to the
caller. This is useful to return multiple rows or columns,
especially with very large result sets. To do this, the function
opens the cursor and returns the cursor name to the caller (or simply
opens the cursor using a portal name specified by or otherwise known
to the caller). The caller can then fetch rows from the cursor. The
cursor can be closed by the caller, or it will be closed automatically
when the transaction closes.
</para>
<para>
The portal name used for a cursor can be specified by the
programmer or automatically generated. To specify a portal name,
simply assign a string to the <type>refcursor</> variable before
opening it. The string value of the <type>refcursor</> variable
will be used by <command>OPEN</> as the name of the underlying portal.
However, if the <type>refcursor</> variable is null,
<command>OPEN</> automatically generates a name that does not
conflict with any existing portal, and assigns it to the
<type>refcursor</> variable.
</para>
<note>
<para>
A bound cursor variable is initialized to the string value
representing its name, so that the portal name is the same as
the cursor variable name, unless the programmer overrides it
by assignment before opening the cursor. But an unbound cursor
variable defaults to the null value initially, so it will receive
an automatically-generated unique name, unless overridden.
</para>
</note>
<para>
The following example shows one way a cursor name can be supplied by
the caller:
<programlisting>
CREATE TABLE test (col text);
INSERT INTO test VALUES ('123');
CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS '
BEGIN
OPEN $1 FOR SELECT col FROM test;
RETURN $1;
END;
' LANGUAGE plpgsql;
BEGIN;
SELECT reffunc('funccursor');
FETCH ALL IN funccursor;
COMMIT;
</programlisting>
</para>
<para>
The following example uses automatic cursor name generation:
<programlisting>
CREATE FUNCTION reffunc2() RETURNS refcursor AS '
DECLARE
ref refcursor;
BEGIN
OPEN ref FOR SELECT col FROM test;
RETURN ref;
END;
' LANGUAGE plpgsql;
BEGIN;
SELECT reffunc2();
reffunc2
--------------------
&lt;unnamed cursor 1&gt;
(1 row)
FETCH ALL IN "&lt;unnamed cursor 1&gt;";
COMMIT;
</programlisting>
</para>
<para>
The following example shows one way to return multiple cursors
from a single function:
<programlisting>
CREATE FUNCTION myfunc(refcursor, refcursor) RETURNS SETOF refcursor AS $$
BEGIN
OPEN $1 FOR SELECT * FROM table_1;
RETURN NEXT $1;
OPEN $2 FOR SELECT * FROM table_2;
RETURN NEXT $2;
END;
$$ LANGUAGE plpgsql;
-- need to be in a transaction to use cursors.
BEGIN;
SELECT * FROM myfunc('a', 'b');
FETCH ALL FROM a;
FETCH ALL FROM b;
COMMIT;
</programlisting>
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-cursor-for-loop">
<title>Looping Through a Cursor's Result</title>
<para>
There is a variant of the <command>FOR</> statement that allows
iterating through the rows returned by a cursor. The syntax is:
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
FOR <replaceable>recordvar</replaceable> IN <replaceable>bound_cursor</replaceable> <optional> ( <replaceable>argument_values</replaceable> ) </optional> LOOP
<replaceable>statements</replaceable>
END LOOP <optional> <replaceable>label</replaceable> </optional>;
</synopsis>
The cursor variable must have been bound to some query when it was
declared, and it <emphasis>cannot</> be open already. The
<command>FOR</> statement automatically opens the cursor, and it closes
the cursor again when the loop exits. A list of actual argument value
expressions must appear if and only if the cursor was declared to take
arguments. These values will be substituted in the query, in just
the same way as during an <command>OPEN</>.
The variable <replaceable>recordvar</replaceable> is automatically
defined as type <type>record</> and exists only inside the loop (any
existing definition of the variable name is ignored within the loop).
Each row returned by the cursor is successively assigned to this
record variable and the loop body is executed.
</para>
</sect2>
</sect1>
<sect1 id="plpgsql-errors-and-messages">
<title>Errors and Messages</title>
<indexterm>
<primary>RAISE</primary>
</indexterm>
<indexterm>
<primary>reporting errors</primary>
<secondary>in PL/PgSQL</secondary>
</indexterm>
<para>
Use the <command>RAISE</command> statement to report messages and
raise errors.
<synopsis>
RAISE <optional> <replaceable class="parameter">level</replaceable> </optional> '<replaceable class="parameter">format</replaceable>' <optional>, <replaceable class="parameter">expression</replaceable> <optional>, ...</optional></optional> <optional> USING <replaceable class="parameter">option</replaceable> = <replaceable class="parameter">expression</replaceable> <optional>, ... </optional> </optional>;
RAISE <optional> <replaceable class="parameter">level</replaceable> </optional> <replaceable class="parameter">condition_name</> <optional> USING <replaceable class="parameter">option</replaceable> = <replaceable class="parameter">expression</replaceable> <optional>, ... </optional> </optional>;
RAISE <optional> <replaceable class="parameter">level</replaceable> </optional> SQLSTATE '<replaceable class="parameter">sqlstate</>' <optional> USING <replaceable class="parameter">option</replaceable> = <replaceable class="parameter">expression</replaceable> <optional>, ... </optional> </optional>;
RAISE <optional> <replaceable class="parameter">level</replaceable> </optional> USING <replaceable class="parameter">option</replaceable> = <replaceable class="parameter">expression</replaceable> <optional>, ... </optional>;
RAISE ;
</synopsis>
The <replaceable class="parameter">level</replaceable> option specifies
the error severity. Allowed levels are <literal>DEBUG</literal>,
<literal>LOG</literal>, <literal>INFO</literal>,
<literal>NOTICE</literal>, <literal>WARNING</literal>,
and <literal>EXCEPTION</literal>, with <literal>EXCEPTION</literal>
being the default.
<literal>EXCEPTION</literal> raises an error (which normally aborts the
current transaction); the other levels only generate messages of different
priority levels.
Whether messages of a particular priority are reported to the client,
written to the server log, or both is controlled by the
<xref linkend="guc-log-min-messages"> and
<xref linkend="guc-client-min-messages"> configuration
variables. See <xref linkend="runtime-config"> for more
information.
</para>
<para>
After <replaceable class="parameter">level</replaceable> if any,
you can write a <replaceable class="parameter">format</replaceable>
(which must be a simple string literal, not an expression). The
format string specifies the error message text to be reported.
The format string can be followed
by optional argument expressions to be inserted into the message.
Inside the format string, <literal>%</literal> is replaced by the
string representation of the next optional argument's value. Write
<literal>%%</literal> to emit a literal <literal>%</literal>.
</para>
<para>
In this example, the value of <literal>v_job_id</> will replace the
<literal>%</literal> in the string:
<programlisting>
RAISE NOTICE 'Calling cs_create_job(%)', v_job_id;
</programlisting>
</para>
<para>
You can attach additional information to the error report by writing
<literal>USING</> followed by <replaceable
class="parameter">option</replaceable> = <replaceable
class="parameter">expression</replaceable> items. The allowed
<replaceable class="parameter">option</replaceable> keywords are
<literal>MESSAGE</>, <literal>DETAIL</>, <literal>HINT</>, and
<literal>ERRCODE</>, while each <replaceable
class="parameter">expression</replaceable> can be any string-valued
expression.
<literal>MESSAGE</> sets the error message text (this option can't
be used in the form of <command>RAISE</> that includes a format
string before <literal>USING</>).
<literal>DETAIL</> supplies an error detail message, while
<literal>HINT</> supplies a hint message.
<literal>ERRCODE</> specifies the error code (SQLSTATE) to report,
either by condition name as shown in <xref linkend="errcodes-appendix">,
or directly as a five-character SQLSTATE code.
</para>
<para>
This example will abort the transaction with the given error message
and hint:
<programlisting>
RAISE EXCEPTION 'Nonexistent ID --> %', user_id USING HINT = 'Please check your user id';
</programlisting>
</para>
<para>
These two examples show equivalent ways of setting the SQLSTATE:
<programlisting>
RAISE 'Duplicate user ID: %', user_id USING ERRCODE = 'unique_violation';
RAISE 'Duplicate user ID: %', user_id USING ERRCODE = '23505';
</programlisting>
</para>
<para>
There is a second <command>RAISE</> syntax in which the main argument
is the condition name or SQLSTATE to be reported, for example:
<programlisting>
RAISE division_by_zero;
RAISE SQLSTATE '22012';
</programlisting>
In this syntax, <literal>USING</> can be used to supply a custom
error message, detail, or hint. Another way to do the earlier
example is
<programlisting>
RAISE unique_violation USING MESSAGE = 'Duplicate user ID: ' || user_id;
</programlisting>
</para>
<para>
Still another variant is to write <literal>RAISE USING</> or <literal>RAISE
<replaceable class="parameter">level</replaceable> USING</> and put
everything else into the <literal>USING</> list.
</para>
<para>
The last variant of <command>RAISE</> has no parameters at all.
This form can only be used inside a <literal>BEGIN</> block's
<literal>EXCEPTION</> clause;
it causes the error currently being handled to be re-thrown to the
next enclosing block.
</para>
<para>
If no condition name nor SQLSTATE is specified in a
<command>RAISE EXCEPTION</command> command, the default is to use
<literal>RAISE_EXCEPTION</> (<literal>P0001</>). If no message
text is specified, the default is to use the condition name or
SQLSTATE as message text.
</para>
<note>
<para>
When specifying an error code by SQLSTATE code, you are not
limited to the predefined error codes, but can select any
error code consisting of five digits and/or upper-case ASCII
letters, other than <literal>00000</>. It is recommended that
you avoid throwing error codes that end in three zeroes, because
these are category codes and can only be trapped by trapping
the whole category.
</para>
</note>
</sect1>
<sect1 id="plpgsql-trigger">
<title>Trigger Procedures</title>
<indexterm zone="plpgsql-trigger">
<primary>trigger</primary>
<secondary>in PL/pgSQL</secondary>
</indexterm>
<para>
<application>PL/pgSQL</application> can be used to define trigger
procedures. A trigger procedure is created with the
<command>CREATE FUNCTION</> command, declaring it as a function with
no arguments and a return type of <type>trigger</type>. Note that
the function must be declared with no arguments even if it expects
to receive arguments specified in <command>CREATE TRIGGER</> &mdash;
trigger arguments are passed via <varname>TG_ARGV</>, as described
below.
</para>
<para>
When a <application>PL/pgSQL</application> function is called as a
trigger, several special variables are created automatically in the
top-level block. They are:
<variablelist>
<varlistentry>
<term><varname>NEW</varname></term>
<listitem>
<para>
Data type <type>RECORD</type>; variable holding the new
database row for <command>INSERT</>/<command>UPDATE</> operations in row-level
triggers. This variable is <symbol>NULL</symbol> in statement-level triggers
and for <command>DELETE</command> operations.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>OLD</varname></term>
<listitem>
<para>
Data type <type>RECORD</type>; variable holding the old
database row for <command>UPDATE</>/<command>DELETE</> operations in row-level
triggers. This variable is <symbol>NULL</symbol> in statement-level triggers
and for <command>INSERT</command> operations.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_NAME</varname></term>
<listitem>
<para>
Data type <type>name</type>; variable that contains the name of the trigger actually
fired.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_WHEN</varname></term>
<listitem>
<para>
Data type <type>text</type>; a string of either
<literal>BEFORE</literal> or <literal>AFTER</literal>
depending on the trigger's definition.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_LEVEL</varname></term>
<listitem>
<para>
Data type <type>text</type>; a string of either
<literal>ROW</literal> or <literal>STATEMENT</literal>
depending on the trigger's definition.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_OP</varname></term>
<listitem>
<para>
Data type <type>text</type>; a string of
<literal>INSERT</literal>, <literal>UPDATE</literal>,
<literal>DELETE</literal>, or <literal>TRUNCATE</>
telling for which operation the trigger was fired.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_RELID</varname></term>
<listitem>
<para>
Data type <type>oid</type>; the object ID of the table that caused the
trigger invocation.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_RELNAME</varname></term>
<listitem>
<para>
Data type <type>name</type>; the name of the table that caused the trigger
invocation. This is now deprecated, and could disappear in a future
release. Use <literal>TG_TABLE_NAME</> instead.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_TABLE_NAME</varname></term>
<listitem>
<para>
Data type <type>name</type>; the name of the table that
caused the trigger invocation.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_TABLE_SCHEMA</varname></term>
<listitem>
<para>
Data type <type>name</type>; the name of the schema of the
table that caused the trigger invocation.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_NARGS</varname></term>
<listitem>
<para>
Data type <type>integer</type>; the number of arguments given to the trigger
procedure in the <command>CREATE TRIGGER</command> statement.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>TG_ARGV[]</varname></term>
<listitem>
<para>
Data type array of <type>text</type>; the arguments from
the <command>CREATE TRIGGER</command> statement.
The index counts from 0. Invalid
indices (less than 0 or greater than or equal to <varname>tg_nargs</>) result in a null value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
A trigger function must return either <symbol>NULL</symbol> or a
record/row value having exactly the structure of the table the
trigger was fired for.
</para>
<para>
Row-level triggers fired <literal>BEFORE</> can return null to signal the
trigger manager to skip the rest of the operation for this row
(i.e., subsequent triggers are not fired, and the
<command>INSERT</>/<command>UPDATE</>/<command>DELETE</> does not occur
for this row). If a nonnull
value is returned then the operation proceeds with that row value.
Returning a row value different from the original value
of <varname>NEW</> alters the row that will be inserted or updated
(but has no direct effect in the <command>DELETE</> case).
To alter the row to be stored, it is possible to replace single values
directly in <varname>NEW</> and return the modified <varname>NEW</>,
or to build a complete new record/row to return.
</para>
<para>
The return value of a <literal>BEFORE</> or <literal>AFTER</>
statement-level trigger or an <literal>AFTER</> row-level trigger is
always ignored; it might as well be null. However, any of these types of
triggers might still abort the entire operation by raising an error.
</para>
<para>
<xref linkend="plpgsql-trigger-example"> shows an example of a
trigger procedure in <application>PL/pgSQL</application>.
</para>
<example id="plpgsql-trigger-example">
<title>A <application>PL/pgSQL</application> Trigger Procedure</title>
<para>
This example trigger ensures that any time a row is inserted or updated
in the table, the current user name and time are stamped into the
row. And it checks that an employee's name is given and that the
salary is a positive value.
</para>
<programlisting>
CREATE TABLE emp (
empname text,
salary integer,
last_date timestamp,
last_user text
);
CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$
BEGIN
-- Check that empname and salary are given
IF NEW.empname IS NULL THEN
RAISE EXCEPTION 'empname cannot be null';
END IF;
IF NEW.salary IS NULL THEN
RAISE EXCEPTION '% cannot have null salary', NEW.empname;
END IF;
-- Who works for us when she must pay for it?
IF NEW.salary &lt; 0 THEN
RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;
END IF;
-- Remember who changed the payroll when
NEW.last_date := current_timestamp;
NEW.last_user := current_user;
RETURN NEW;
END;
$emp_stamp$ LANGUAGE plpgsql;
CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
FOR EACH ROW EXECUTE PROCEDURE emp_stamp();
</programlisting>
</example>
<para>
Another way to log changes to a table involves creating a new table that
holds a row for each insert, update, or delete that occurs. This approach
can be thought of as auditing changes to a table.
<xref linkend="plpgsql-trigger-audit-example"> shows an example of an
audit trigger procedure in <application>PL/pgSQL</application>.
</para>
<example id="plpgsql-trigger-audit-example">
<title>A <application>PL/pgSQL</application> Trigger Procedure For Auditing</title>
<para>
This example trigger ensures that any insert, update or delete of a row
in the <literal>emp</literal> table is recorded (i.e., audited) in the <literal>emp_audit</literal> table.
The current time and user name are stamped into the row, together with
the type of operation performed on it.
</para>
<programlisting>
CREATE TABLE emp (
empname text NOT NULL,
salary integer
);
CREATE TABLE emp_audit(
operation char(1) NOT NULL,
stamp timestamp NOT NULL,
userid text NOT NULL,
empname text NOT NULL,
salary integer
);
CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
BEGIN
--
-- Create a row in emp_audit to reflect the operation performed on emp,
-- make use of the special variable TG_OP to work out the operation.
--
IF (TG_OP = 'DELETE') THEN
INSERT INTO emp_audit SELECT 'D', now(), user, OLD.*;
RETURN OLD;
ELSIF (TG_OP = 'UPDATE') THEN
INSERT INTO emp_audit SELECT 'U', now(), user, NEW.*;
RETURN NEW;
ELSIF (TG_OP = 'INSERT') THEN
INSERT INTO emp_audit SELECT 'I', now(), user, NEW.*;
RETURN NEW;
END IF;
RETURN NULL; -- result is ignored since this is an AFTER trigger
END;
$emp_audit$ LANGUAGE plpgsql;
CREATE TRIGGER emp_audit
AFTER INSERT OR UPDATE OR DELETE ON emp
FOR EACH ROW EXECUTE PROCEDURE process_emp_audit();
</programlisting>
</example>
<para>
One use of triggers is to maintain a summary table
of another table. The resulting summary can be used in place of the
original table for certain queries &mdash; often with vastly reduced run
times.
This technique is commonly used in Data Warehousing, where the tables
of measured or observed data (called fact tables) might be extremely large.
<xref linkend="plpgsql-trigger-summary-example"> shows an example of a
trigger procedure in <application>PL/pgSQL</application> that maintains
a summary table for a fact table in a data warehouse.
</para>
<example id="plpgsql-trigger-summary-example">
<title>A <application>PL/pgSQL</application> Trigger Procedure For Maintaining A Summary Table</title>
<para>
The schema detailed here is partly based on the <emphasis>Grocery Store
</emphasis> example from <emphasis>The Data Warehouse Toolkit</emphasis>
by Ralph Kimball.
</para>
<programlisting>
--
-- Main tables - time dimension and sales fact.
--
CREATE TABLE time_dimension (
time_key integer NOT NULL,
day_of_week integer NOT NULL,
day_of_month integer NOT NULL,
month integer NOT NULL,
quarter integer NOT NULL,
year integer NOT NULL
);
CREATE UNIQUE INDEX time_dimension_key ON time_dimension(time_key);
CREATE TABLE sales_fact (
time_key integer NOT NULL,
product_key integer NOT NULL,
store_key integer NOT NULL,
amount_sold numeric(12,2) NOT NULL,
units_sold integer NOT NULL,
amount_cost numeric(12,2) NOT NULL
);
CREATE INDEX sales_fact_time ON sales_fact(time_key);
--
-- Summary table - sales by time.
--
CREATE TABLE sales_summary_bytime (
time_key integer NOT NULL,
amount_sold numeric(15,2) NOT NULL,
units_sold numeric(12) NOT NULL,
amount_cost numeric(15,2) NOT NULL
);
CREATE UNIQUE INDEX sales_summary_bytime_key ON sales_summary_bytime(time_key);
--
-- Function and trigger to amend summarized column(s) on UPDATE, INSERT, DELETE.
--
CREATE OR REPLACE FUNCTION maint_sales_summary_bytime() RETURNS TRIGGER AS $maint_sales_summary_bytime$
DECLARE
delta_time_key integer;
delta_amount_sold numeric(15,2);
delta_units_sold numeric(12);
delta_amount_cost numeric(15,2);
BEGIN
-- Work out the increment/decrement amount(s).
IF (TG_OP = 'DELETE') THEN
delta_time_key = OLD.time_key;
delta_amount_sold = -1 * OLD.amount_sold;
delta_units_sold = -1 * OLD.units_sold;
delta_amount_cost = -1 * OLD.amount_cost;
ELSIF (TG_OP = 'UPDATE') THEN
-- forbid updates that change the time_key -
-- (probably not too onerous, as DELETE + INSERT is how most
-- changes will be made).
IF ( OLD.time_key != NEW.time_key) THEN
RAISE EXCEPTION 'Update of time_key : % -&gt; % not allowed', OLD.time_key, NEW.time_key;
END IF;
delta_time_key = OLD.time_key;
delta_amount_sold = NEW.amount_sold - OLD.amount_sold;
delta_units_sold = NEW.units_sold - OLD.units_sold;
delta_amount_cost = NEW.amount_cost - OLD.amount_cost;
ELSIF (TG_OP = 'INSERT') THEN
delta_time_key = NEW.time_key;
delta_amount_sold = NEW.amount_sold;
delta_units_sold = NEW.units_sold;
delta_amount_cost = NEW.amount_cost;
END IF;
-- Insert or update the summary row with the new values.
&lt;&lt;insert_update&gt;&gt;
LOOP
UPDATE sales_summary_bytime
SET amount_sold = amount_sold + delta_amount_sold,
units_sold = units_sold + delta_units_sold,
amount_cost = amount_cost + delta_amount_cost
WHERE time_key = delta_time_key;
EXIT insert_update WHEN found;
BEGIN
INSERT INTO sales_summary_bytime (
time_key,
amount_sold,
units_sold,
amount_cost)
VALUES (
delta_time_key,
delta_amount_sold,
delta_units_sold,
delta_amount_cost
);
EXIT insert_update;
EXCEPTION
WHEN UNIQUE_VIOLATION THEN
-- do nothing
END;
END LOOP insert_update;
RETURN NULL;
END;
$maint_sales_summary_bytime$ LANGUAGE plpgsql;
CREATE TRIGGER maint_sales_summary_bytime
AFTER INSERT OR UPDATE OR DELETE ON sales_fact
FOR EACH ROW EXECUTE PROCEDURE maint_sales_summary_bytime();
INSERT INTO sales_fact VALUES(1,1,1,10,3,15);
INSERT INTO sales_fact VALUES(1,2,1,20,5,35);
INSERT INTO sales_fact VALUES(2,2,1,40,15,135);
INSERT INTO sales_fact VALUES(2,3,1,10,1,13);
SELECT * FROM sales_summary_bytime;
DELETE FROM sales_fact WHERE product_key = 1;
SELECT * FROM sales_summary_bytime;
UPDATE sales_fact SET units_sold = units_sold * 2;
SELECT * FROM sales_summary_bytime;
</programlisting>
</example>
</sect1>
<sect1 id="plpgsql-implementation">
<title><application>PL/pgSQL</> Under the Hood</title>
<para>
This section discusses some implementation details that are
frequently important for <application>PL/pgSQL</> users to know.
</para>
<sect2 id="plpgsql-var-subst">
<title>Variable Substitution</title>
<para>
When <application>PL/pgSQL</> prepares a SQL statement or expression
for execution, any <application>PL/pgSQL</application> variable name
appearing in the statement or expression is replaced by a parameter symbol,
<literal>$<replaceable>n</replaceable></literal>. The current value
of the variable is then provided as the value for the parameter whenever
the statement or expression is executed. As an example, consider the
function
<programlisting>
CREATE FUNCTION logfunc(logtxt text) RETURNS void AS $$
DECLARE
curtime timestamp := now();
BEGIN
INSERT INTO logtable VALUES (logtxt, curtime);
END;
$$ LANGUAGE plpgsql;
</programlisting>
The <command>INSERT</> statement will effectively be processed as
<programlisting>
PREPARE <replaceable>statement_name</>(text, timestamp) AS
INSERT INTO logtable VALUES ($1, $2);
</programlisting>
followed on each execution by <command>EXECUTE</> with the current
actual values of the two variables. (Note: here we are speaking of
the main SQL engine's
<xref linkend="sql-execute" endterm="sql-execute-title"> command,
not <application>PL/pgSQL</application>'s <command>EXECUTE</>.)
</para>
<para>
<emphasis>The substitution mechanism will replace any token that matches a
known variable's name.</> This poses various traps for the unwary.
For example, it is a bad idea
to use a variable name that is the same as any table or column name
that you need to reference in queries within the function, because
what you think is a table or column name will still get replaced.
In the above example, suppose that <structname>logtable</> has
column names <structfield>logtxt</> and <structfield>logtime</>,
and we try to write the <command>INSERT</> as
<programlisting>
INSERT INTO logtable (logtxt, logtime) VALUES (logtxt, curtime);
</programlisting>
This will be fed to the main SQL parser as
<programlisting>
INSERT INTO logtable ($1, logtime) VALUES ($1, $2);
</programlisting>
resulting in a syntax error like this:
<screen>
ERROR: syntax error at or near "$1"
LINE 1: INSERT INTO logtable ( $1 , logtime) VALUES ( $1 , $2 )
^
QUERY: INSERT INTO logtable ( $1 , logtime) VALUES ( $1 , $2 )
CONTEXT: SQL statement in PL/PgSQL function "logfunc2" near line 5
</screen>
</para>
<para>
This example is fairly easy to diagnose, since it leads to an
obvious syntax error. Much nastier are cases where the substitution
is syntactically permissible, since the only symptom may be misbehavior
of the function. In one case, a user wrote something like this:
<programlisting>
DECLARE
val text;
search_key integer;
BEGIN
...
FOR val IN SELECT val FROM table WHERE key = search_key LOOP ...
</programlisting>
and wondered why all his table entries seemed to be NULL. Of course
what happened here was that the query became
<programlisting>
SELECT $1 FROM table WHERE key = $2
</programlisting>
and thus it was just an expensive way of assigning <literal>val</>'s
current value back to itself for each row.
</para>
<para>
A commonly used coding rule for avoiding such traps is to use a
different naming convention for <application>PL/pgSQL</application>
variables than you use for table and column names. For example,
if all your variables are named
<literal>v_<replaceable>something</></literal> while none of your
table or column names start with <literal>v_</>, you're pretty safe.
</para>
<para>
Another workaround is to use qualified (dotted) names for SQL entities.
For instance we could safely have written the above example as
<programlisting>
FOR val IN SELECT table.val FROM table WHERE key = search_key LOOP ...
</programlisting>
because <application>PL/pgSQL</application> will not substitute a
variable for a trailing component of a qualified name.
However this solution does not work in every case &mdash; you can't
qualify a name in an <command>INSERT</>'s column name list, for instance.
Another point is that record and row variable names will be matched to
the first components of qualified names, so a qualified SQL name is
still vulnerable in some cases.
In such cases choosing a non-conflicting variable name is the only way.
</para>
<para>
Another technique you can use is to attach a label to the block in
which your variables are declared, and then qualify the variable names
in your SQL commands (see <xref linkend="plpgsql-structure">).
For example,
<programlisting>
&lt;&lt;pl&gt;&gt;
DECLARE
val text;
BEGIN
...
UPDATE table SET col = pl.val WHERE ...
</programlisting>
This is not in itself a solution to the problem of conflicts,
since an unqualified name in a SQL command is still at risk of being
interpreted the <quote>wrong</> way. But it is useful for clarifying
the intent of potentially-ambiguous code.
</para>
<para>
Variable substitution does not happen in the command string given
to <command>EXECUTE</> or one of its variants. If you need to
insert a varying value into such a command, do so as part of
constructing the string value, as illustrated in
<xref linkend="plpgsql-statements-executing-dyn">.
</para>
<para>
Variable substitution currently works only in <command>SELECT</>,
<command>INSERT</>, <command>UPDATE</>, and <command>DELETE</> commands,
because the main SQL engine allows parameter symbols only in these
commands. To use a non-constant name or value in other statement
types (generically called utility statements), you must construct
the utility statement as a string and <command>EXECUTE</> it.
</para>
</sect2>
<sect2 id="plpgsql-plan-caching">
<title>Plan Caching</title>
<para>
The <application>PL/pgSQL</> interpreter parses the function's source
text and produces an internal binary instruction tree the first time the
function is called (within each session). The instruction tree
fully translates the
<application>PL/pgSQL</> statement structure, but individual
<acronym>SQL</acronym> expressions and <acronym>SQL</acronym> commands
used in the function are not translated immediately.
</para>
<para>
As each expression and <acronym>SQL</acronym> command is first
executed in the function, the <application>PL/pgSQL</> interpreter
creates a prepared execution plan (using the
<acronym>SPI</acronym> manager's <function>SPI_prepare</function>
and <function>SPI_saveplan</function>
functions).<indexterm><primary>preparing a query</><secondary>in
PL/pgSQL</></> Subsequent visits to that expression or command
reuse the prepared plan. Thus, a function with conditional code
that contains many statements for which execution plans might be
required will only prepare and save those plans that are really
used during the lifetime of the database connection. This can
substantially reduce the total amount of time required to parse
and generate execution plans for the statements in a
<application>PL/pgSQL</> function. A disadvantage is that errors
in a specific expression or command cannot be detected until that
part of the function is reached in execution. (Trivial syntax
errors will be detected during the initial parsing pass, but
anything deeper will not be detected until execution.)
</para>
<para>
Once <application>PL/pgSQL</> has made an execution plan for a particular
command in a function, it will reuse that plan for the life of the
database connection. This is usually a win for performance, but it
can cause some problems if you dynamically
alter your database schema. For example:
<programlisting>
CREATE FUNCTION populate() RETURNS integer AS $$
DECLARE
-- declarations
BEGIN
PERFORM my_function();
END;
$$ LANGUAGE plpgsql;
</programlisting>
If you execute the above function, it will reference the OID for
<function>my_function()</function> in the execution plan produced for
the <command>PERFORM</command> statement. Later, if you
drop and recreate <function>my_function()</function>, then
<function>populate()</function> will not be able to find
<function>my_function()</function> anymore. You would then have to
start a new database session so that <function>populate()</function>
will be compiled afresh, before it will work again. You can avoid
this problem by using <command>CREATE OR REPLACE FUNCTION</command>
when updating the definition of
<function>my_function</function>, since when a function is
<quote>replaced</quote>, its OID is not changed.
</para>
<note>
<para>
In <productname>PostgreSQL</productname> 8.3 and later, saved plans
will be replaced whenever any schema changes have occurred to any
tables they reference. This eliminates one of the major disadvantages
of saved plans. However, there is no such mechanism for function
references, and thus the above example involving a reference to a
deleted function is still valid.
</para>
</note>
<para>
Because <application>PL/pgSQL</application> saves execution plans
in this way, SQL commands that appear directly in a
<application>PL/pgSQL</application> function must refer to the
same tables and columns on every execution; that is, you cannot use
a parameter as the name of a table or column in an SQL command. To get
around this restriction, you can construct dynamic commands using
the <application>PL/pgSQL</application> <command>EXECUTE</command>
statement &mdash; at the price of constructing a new execution plan on
every execution.
</para>
<para>
Another important point is that the prepared plans are parameterized
to allow the values of <application>PL/pgSQL</application> variables
to change from one use to the next, as discussed in detail above.
Sometimes this means that a plan is less efficient than it would be
if generated for a specific variable value. As an example, consider
<programlisting>
SELECT * INTO myrec FROM dictionary WHERE word LIKE search_term;
</programlisting>
where <literal>search_term</> is a <application>PL/pgSQL</application>
variable. The cached plan for this query will never use an index on
<structfield>word</>, since the planner cannot assume that the
<literal>LIKE</> pattern will be left-anchored at run time. To use
an index the query must be planned with a specific constant
<literal>LIKE</> pattern provided. This is another situation where
<command>EXECUTE</command> can be used to force a new plan to be
generated for each execution.
</para>
<para>
The mutable nature of record variables presents another problem in this
connection. When fields of a record variable are used in
expressions or statements, the data types of the fields must not
change from one call of the function to the next, since each
expression will be planned using the data type that is present
when the expression is first reached. <command>EXECUTE</command> can be
used to get around this problem when necessary.
</para>
<para>
If the same function is used as a trigger for more than one table,
<application>PL/pgSQL</application> prepares and caches plans
independently for each such table &mdash; that is, there is a cache
for each trigger function and table combination, not just for each
function. This alleviates some of the problems with varying
data types; for instance, a trigger function will be able to work
successfully with a column named <literal>key</> even if it happens
to have different types in different tables.
</para>
<para>
Likewise, functions having polymorphic argument types have a separate
plan cache for each combination of actual argument types they have been
invoked for, so that data type differences do not cause unexpected
failures.
</para>
<para>
Plan caching can sometimes have surprising effects on the interpretation
of time-sensitive values. For example there
is a difference between what these two functions do:
<programlisting>
CREATE FUNCTION logfunc1(logtxt text) RETURNS void AS $$
BEGIN
INSERT INTO logtable VALUES (logtxt, 'now');
END;
$$ LANGUAGE plpgsql;
</programlisting>
and:
<programlisting>
CREATE FUNCTION logfunc2(logtxt text) RETURNS void AS $$
DECLARE
curtime timestamp;
BEGIN
curtime := 'now';
INSERT INTO logtable VALUES (logtxt, curtime);
END;
$$ LANGUAGE plpgsql;
</programlisting>
</para>
<para>
In the case of <function>logfunc1</function>, the
<productname>PostgreSQL</productname> main parser knows when
preparing the plan for the <command>INSERT</command> that the
string <literal>'now'</literal> should be interpreted as
<type>timestamp</type>, because the target column of
<classname>logtable</classname> is of that type. Thus,
<literal>'now'</literal> will be converted to a constant when the
<command>INSERT</command> is planned, and then used in all
invocations of <function>logfunc1</function> during the lifetime
of the session. Needless to say, this isn't what the programmer
wanted.
</para>
<para>
In the case of <function>logfunc2</function>, the
<productname>PostgreSQL</productname> main parser does not know
what type <literal>'now'</literal> should become and therefore
it returns a data value of type <type>text</type> containing the string
<literal>now</literal>. During the ensuing assignment
to the local variable <varname>curtime</varname>, the
<application>PL/pgSQL</application> interpreter casts this
string to the <type>timestamp</type> type by calling the
<function>text_out</function> and <function>timestamp_in</function>
functions for the conversion. So, the computed time stamp is updated
on each execution as the programmer expects.
</para>
</sect2>
</sect1>
<sect1 id="plpgsql-development-tips">
<title>Tips for Developing in <application>PL/pgSQL</application></title>
<para>
One good way to develop in
<application>PL/pgSQL</> is to use the text editor of your
choice to create your functions, and in another window, use
<application>psql</application> to load and test those functions.
If you are doing it this way, it
is a good idea to write the function using <command>CREATE OR
REPLACE FUNCTION</>. That way you can just reload the file to update
the function definition. For example:
<programlisting>
CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $$
....
$$ LANGUAGE plpgsql;
</programlisting>
</para>
<para>
While running <application>psql</application>, you can load or reload such
a function definition file with:
<programlisting>
\i filename.sql
</programlisting>
and then immediately issue SQL commands to test the function.
</para>
<para>
Another good way to develop in <application>PL/pgSQL</> is with a
GUI database access tool that facilitates development in a
procedural language. One example of such as a tool is
<application>pgAdmin</>, although others exist. These tools often
provide convenient features such as escaping single quotes and
making it easier to recreate and debug functions.
</para>
<sect2 id="plpgsql-quote-tips">
<title>Handling of Quotation Marks</title>
<para>
The code of a <application>PL/pgSQL</> function is specified in
<command>CREATE FUNCTION</command> as a string literal. If you
write the string literal in the ordinary way with surrounding
single quotes, then any single quotes inside the function body
must be doubled; likewise any backslashes must be doubled (assuming
escape string syntax is used).
Doubling quotes is at best tedious, and in more complicated cases
the code can become downright incomprehensible, because you can
easily find yourself needing half a dozen or more adjacent quote marks.
It's recommended that you instead write the function body as a
<quote>dollar-quoted</> string literal (see <xref
linkend="sql-syntax-dollar-quoting">). In the dollar-quoting
approach, you never double any quote marks, but instead take care to
choose a different dollar-quoting delimiter for each level of
nesting you need. For example, you might write the <command>CREATE
FUNCTION</command> command as:
<programlisting>
CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $PROC$
....
$PROC$ LANGUAGE plpgsql;
</programlisting>
Within this, you might use quote marks for simple literal strings in
SQL commands and <literal>$$</> to delimit fragments of SQL commands
that you are assembling as strings. If you need to quote text that
includes <literal>$$</>, you could use <literal>$Q$</>, and so on.
</para>
<para>
The following chart shows what you have to do when writing quote
marks without dollar quoting. It might be useful when translating
pre-dollar quoting code into something more comprehensible.
</para>
<variablelist>
<varlistentry>
<term>1 quotation mark</term>
<listitem>
<para>
To begin and end the function body, for example:
<programlisting>
CREATE FUNCTION foo() RETURNS integer AS '
....
' LANGUAGE plpgsql;
</programlisting>
Anywhere within a single-quoted function body, quote marks
<emphasis>must</> appear in pairs.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>2 quotation marks</term>
<listitem>
<para>
For string literals inside the function body, for example:
<programlisting>
a_output := ''Blah'';
SELECT * FROM users WHERE f_name=''foobar'';
</programlisting>
In the dollar-quoting approach, you'd just write:
<programlisting>
a_output := 'Blah';
SELECT * FROM users WHERE f_name='foobar';
</programlisting>
which is exactly what the <application>PL/pgSQL</> parser would see
in either case.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>4 quotation marks</term>
<listitem>
<para>
When you need a single quotation mark in a string constant inside the
function body, for example:
<programlisting>
a_output := a_output || '' AND name LIKE ''''foobar'''' AND xyz''
</programlisting>
The value actually appended to <literal>a_output</literal> would be:
<literal> AND name LIKE 'foobar' AND xyz</literal>.
</para>
<para>
In the dollar-quoting approach, you'd write:
<programlisting>
a_output := a_output || $$ AND name LIKE 'foobar' AND xyz$$
</programlisting>
being careful that any dollar-quote delimiters around this are not
just <literal>$$</>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>6 quotation marks</term>
<listitem>
<para>
When a single quotation mark in a string inside the function body is
adjacent to the end of that string constant, for example:
<programlisting>
a_output := a_output || '' AND name LIKE ''''foobar''''''
</programlisting>
The value appended to <literal>a_output</literal> would then be:
<literal> AND name LIKE 'foobar'</literal>.
</para>
<para>
In the dollar-quoting approach, this becomes:
<programlisting>
a_output := a_output || $$ AND name LIKE 'foobar'$$
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>10 quotation marks</term>
<listitem>
<para>
When you want two single quotation marks in a string constant (which
accounts for 8 quotation marks) and this is adjacent to the end of that
string constant (2 more). You will probably only need that if
you are writing a function that generates other functions, as in
<xref linkend="plpgsql-porting-ex2">.
For example:
<programlisting>
a_output := a_output || '' if v_'' ||
referrer_keys.kind || '' like ''''''''''
|| referrer_keys.key_string || ''''''''''
then return '''''' || referrer_keys.referrer_type
|| ''''''; end if;'';
</programlisting>
The value of <literal>a_output</literal> would then be:
<programlisting>
if v_... like ''...'' then return ''...''; end if;
</programlisting>
</para>
<para>
In the dollar-quoting approach, this becomes:
<programlisting>
a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
|| referrer_keys.key_string || $$'
then return '$$ || referrer_keys.referrer_type
|| $$'; end if;$$;
</programlisting>
where we assume we only need to put single quote marks into
<literal>a_output</literal>, because it will be re-quoted before use.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<!-- **** Porting from Oracle PL/SQL **** -->
<sect1 id="plpgsql-porting">
<title>Porting from <productname>Oracle</productname> PL/SQL</title>
<indexterm zone="plpgsql-porting">
<primary>Oracle</primary>
<secondary>porting from PL/SQL to PL/pgSQL</secondary>
</indexterm>
<indexterm zone="plpgsql-porting">
<primary>PL/SQL (Oracle)</primary>
<secondary>porting to PL/pgSQL</secondary>
</indexterm>
<para>
This section explains differences between
<productname>PostgreSQL</>'s <application>PL/pgSQL</application>
language and Oracle's <application>PL/SQL</application> language,
to help developers who port applications from
<trademark class=registered>Oracle</> to <productname>PostgreSQL</>.
</para>
<para>
<application>PL/pgSQL</application> is similar to PL/SQL in many
aspects. It is a block-structured, imperative language, and all
variables have to be declared. Assignments, loops, conditionals
are similar. The main differences you should keep in mind when
porting from <application>PL/SQL</> to
<application>PL/pgSQL</application> are:
<itemizedlist>
<listitem>
<para>
There are no default values for parameters in <productname>PostgreSQL</>.
</para>
</listitem>
<listitem>
<para>
You can overload function names in <productname>PostgreSQL</>. This is
often used to work around the lack of default parameters.
</para>
</listitem>
<listitem>
<para>
If a name used in a SQL command could be either a column name of a
table or a reference to a variable of the function,
<application>PL/SQL</> treats it as a column name, while
<application>PL/pgSQL</> treats it as a variable name. It's best
to avoid such ambiguities in the first place, but if necessary you
can fix them by properly qualifying the ambiguous name.
(See <xref linkend="plpgsql-var-subst">.)
</para>
</listitem>
<listitem>
<para>
In <productname>PostgreSQL</> the function body must be written as
a string literal. Therefore you need to use dollar quoting or escape
single quotes in the function body. (See <xref
linkend="plpgsql-quote-tips">.)
</para>
</listitem>
<listitem>
<para>
Instead of packages, use schemas to organize your functions
into groups.
</para>
</listitem>
<listitem>
<para>
Since there are no packages, there are no package-level variables
either. This is somewhat annoying. You can keep per-session state
in temporary tables instead.
</para>
</listitem>
<listitem>
<para>
Integer <command>FOR</> loops with <literal>REVERSE</> work
differently: <application>PL/SQL</> counts down from the second
number to the first, while <application>PL/pgSQL</> counts down
from the first number to the second, requiring the loop bounds
to be swapped when porting. This incompatibility is unfortunate
but is unlikely to be changed. (See <xref
linkend="plpgsql-integer-for">.)
</para>
</listitem>
<listitem>
<para>
<command>FOR</> loops over queries (other than cursors) also work
differently: the target variable(s) must have been declared,
whereas <application>PL/SQL</> always declares them implicitly.
An advantage of this is that the variable values are still accessible
after the loop exits.
</para>
</listitem>
<listitem>
<para>
There are various notational differences for the use of cursor
variables.
</para>
</listitem>
</itemizedlist>
</para>
<sect2>
<title>Porting Examples</title>
<para>
<xref linkend="pgsql-porting-ex1"> shows how to port a simple
function from <application>PL/SQL</> to <application>PL/pgSQL</>.
</para>
<example id="pgsql-porting-ex1">
<title>Porting a Simple Function from <application>PL/SQL</> to <application>PL/pgSQL</></title>
<para>
Here is an <productname>Oracle</productname> <application>PL/SQL</> function:
<programlisting>
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar,
v_version varchar)
RETURN varchar IS
BEGIN
IF v_version IS NULL THEN
RETURN v_name;
END IF;
RETURN v_name || '/' || v_version;
END;
/
show errors;
</programlisting>
</para>
<para>
Let's go through this function and see the differences compared to
<application>PL/pgSQL</>:
<itemizedlist>
<listitem>
<para>
The <literal>RETURN</literal> key word in the function
prototype (not the function body) becomes
<literal>RETURNS</literal> in
<productname>PostgreSQL</productname>.
Also, <literal>IS</> becomes <literal>AS</>, and you need to
add a <literal>LANGUAGE</> clause because <application>PL/pgSQL</>
is not the only possible function language.
</para>
</listitem>
<listitem>
<para>
In <productname>PostgreSQL</>, the function body is considered
to be a string literal, so you need to use quote marks or dollar
quotes around it. This substitutes for the terminating <literal>/</>
in the Oracle approach.
</para>
</listitem>
<listitem>
<para>
The <literal>show errors</literal> command does not exist in
<productname>PostgreSQL</>, and is not needed since errors are
reported automatically.
</para>
</listitem>
</itemizedlist>
</para>
<para>
This is how this function would look when ported to
<productname>PostgreSQL</>:
<programlisting>
CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar,
v_version varchar)
RETURNS varchar AS $$
BEGIN
IF v_version IS NULL THEN
RETURN v_name;
END IF;
RETURN v_name || '/' || v_version;
END;
$$ LANGUAGE plpgsql;
</programlisting>
</para>
</example>
<para>
<xref linkend="plpgsql-porting-ex2"> shows how to port a
function that creates another function and how to handle the
ensuing quoting problems.
</para>
<example id="plpgsql-porting-ex2">
<title>Porting a Function that Creates Another Function from <application>PL/SQL</> to <application>PL/pgSQL</></title>
<para>
The following procedure grabs rows from a
<command>SELECT</command> statement and builds a large function
with the results in <literal>IF</literal> statements, for the
sake of efficiency.
</para>
<para>
This is the Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc IS
CURSOR referrer_keys IS
SELECT * FROM cs_referrer_keys
ORDER BY try_order;
func_cmd VARCHAR(4000);
BEGIN
func_cmd := 'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host IN VARCHAR,
v_domain IN VARCHAR, v_url IN VARCHAR) RETURN VARCHAR IS BEGIN';
FOR referrer_key IN referrer_keys LOOP
func_cmd := func_cmd ||
' IF v_' || referrer_key.kind
|| ' LIKE ''' || referrer_key.key_string
|| ''' THEN RETURN ''' || referrer_key.referrer_type
|| '''; END IF;';
END LOOP;
func_cmd := func_cmd || ' RETURN NULL; END;';
EXECUTE IMMEDIATE func_cmd;
END;
/
show errors;
</programlisting>
</para>
<para>
Here is how this function would end up in <productname>PostgreSQL</>:
<programlisting>
CREATE OR REPLACE FUNCTION cs_update_referrer_type_proc() RETURNS void AS $func$
DECLARE
CURSOR referrer_keys IS
SELECT * FROM cs_referrer_keys
ORDER BY try_order;
func_body text;
func_cmd text;
BEGIN
func_body := 'BEGIN';
FOR referrer_key IN referrer_keys LOOP
func_body := func_body ||
' IF v_' || referrer_key.kind
|| ' LIKE ' || quote_literal(referrer_key.key_string)
|| ' THEN RETURN ' || quote_literal(referrer_key.referrer_type)
|| '; END IF;' ;
END LOOP;
func_body := func_body || ' RETURN NULL; END;';
func_cmd :=
'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host varchar,
v_domain varchar,
v_url varchar)
RETURNS varchar AS '
|| quote_literal(func_body)
|| ' LANGUAGE plpgsql;' ;
EXECUTE func_cmd;
END;
$func$ LANGUAGE plpgsql;
</programlisting>
Notice how the body of the function is built separately and passed
through <literal>quote_literal</> to double any quote marks in it. This
technique is needed because we cannot safely use dollar quoting for
defining the new function: we do not know for sure what strings will
be interpolated from the <structfield>referrer_key.key_string</> field.
(We are assuming here that <structfield>referrer_key.kind</> can be
trusted to always be <literal>host</>, <literal>domain</>, or
<literal>url</>, but <structfield>referrer_key.key_string</> might be
anything, in particular it might contain dollar signs.) This function
is actually an improvement on the Oracle original, because it will
not generate broken code when <structfield>referrer_key.key_string</> or
<structfield>referrer_key.referrer_type</> contain quote marks.
</para>
</example>
<para>
<xref linkend="plpgsql-porting-ex3"> shows how to port a function
with <literal>OUT</> parameters and string manipulation.
<productname>PostgreSQL</> does not have a built-in
<function>instr</function> function, but you can create one
using a combination of other
functions.<indexterm><primary>instr</></indexterm> In <xref
linkend="plpgsql-porting-appendix"> there is a
<application>PL/pgSQL</application> implementation of
<function>instr</function> that you can use to make your porting
easier.
</para>
<example id="plpgsql-porting-ex3">
<title>Porting a Procedure With String Manipulation and
<literal>OUT</> Parameters from <application>PL/SQL</> to
<application>PL/pgSQL</></title>
<para>
The following <productname>Oracle</productname> PL/SQL procedure is used
to parse a URL and return several elements (host, path, and query).
</para>
<para>
This is the Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_parse_url(
v_url IN VARCHAR,
v_host OUT VARCHAR, -- This will be passed back
v_path OUT VARCHAR, -- This one too
v_query OUT VARCHAR) -- And this one
IS
a_pos1 INTEGER;
a_pos2 INTEGER;
BEGIN
v_host := NULL;
v_path := NULL;
v_query := NULL;
a_pos1 := instr(v_url, '//');
IF a_pos1 = 0 THEN
RETURN;
END IF;
a_pos2 := instr(v_url, '/', a_pos1 + 2);
IF a_pos2 = 0 THEN
v_host := substr(v_url, a_pos1 + 2);
v_path := '/';
RETURN;
END IF;
v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
a_pos1 := instr(v_url, '?', a_pos2 + 1);
IF a_pos1 = 0 THEN
v_path := substr(v_url, a_pos2);
RETURN;
END IF;
v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
v_query := substr(v_url, a_pos1 + 1);
END;
/
show errors;
</programlisting>
</para>
<para>
Here is a possible translation into <application>PL/pgSQL</>:
<programlisting>
CREATE OR REPLACE FUNCTION cs_parse_url(
v_url IN VARCHAR,
v_host OUT VARCHAR, -- This will be passed back
v_path OUT VARCHAR, -- This one too
v_query OUT VARCHAR) -- And this one
AS $$
DECLARE
a_pos1 INTEGER;
a_pos2 INTEGER;
BEGIN
v_host := NULL;
v_path := NULL;
v_query := NULL;
a_pos1 := instr(v_url, '//');
IF a_pos1 = 0 THEN
RETURN;
END IF;
a_pos2 := instr(v_url, '/', a_pos1 + 2);
IF a_pos2 = 0 THEN
v_host := substr(v_url, a_pos1 + 2);
v_path := '/';
RETURN;
END IF;
v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
a_pos1 := instr(v_url, '?', a_pos2 + 1);
IF a_pos1 = 0 THEN
v_path := substr(v_url, a_pos2);
RETURN;
END IF;
v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
v_query := substr(v_url, a_pos1 + 1);
END;
$$ LANGUAGE plpgsql;
</programlisting>
This function could be used like this:
<programlisting>
SELECT * FROM cs_parse_url('http://foobar.com/query.cgi?baz');
</programlisting>
</para>
</example>
<para>
<xref linkend="plpgsql-porting-ex4"> shows how to port a procedure
that uses numerous features that are specific to Oracle.
</para>
<example id="plpgsql-porting-ex4">
<title>Porting a Procedure from <application>PL/SQL</> to <application>PL/pgSQL</></title>
<para>
The Oracle version:
<programlisting>
CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id IN INTEGER) IS
a_running_job_count INTEGER;
PRAGMA AUTONOMOUS_TRANSACTION;<co id="co.plpgsql-porting-pragma">
BEGIN
LOCK TABLE cs_jobs IN EXCLUSIVE MODE;<co id="co.plpgsql-porting-locktable">
SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;
IF a_running_job_count &gt; 0 THEN
COMMIT; -- free lock<co id="co.plpgsql-porting-commit">
raise_application_error(-20000, 'Unable to create a new job: a job is currently running.');
END IF;
DELETE FROM cs_active_job;
INSERT INTO cs_active_job(job_id) VALUES (v_job_id);
BEGIN
INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, sysdate);
EXCEPTION
WHEN dup_val_on_index THEN NULL; -- don't worry if it already exists
END;
COMMIT;
END;
/
show errors
</programlisting>
</para>
<para>
Procedures like this can easily be converted into <productname>PostgreSQL</>
functions returning <type>void</type>. This procedure in
particular is interesting because it can teach us some things:
<calloutlist>
<callout arearefs="co.plpgsql-porting-pragma">
<para>
There is no <literal>PRAGMA</literal> statement in <productname>PostgreSQL</>.
</para>
</callout>
<callout arearefs="co.plpgsql-porting-locktable">
<para>
If you do a <command>LOCK TABLE</command> in <application>PL/pgSQL</>,
the lock will not be released until the calling transaction is
finished.
</para>
</callout>
<callout arearefs="co.plpgsql-porting-commit">
<para>
You cannot issue <command>COMMIT</> in a
<application>PL/pgSQL</application> function. The function is
running within some outer transaction and so <command>COMMIT</>
would imply terminating the function's execution. However, in
this particular case it is not necessary anyway, because the lock
obtained by the <command>LOCK TABLE</command> will be released when
we raise an error.
</para>
</callout>
</calloutlist>
</para>
<para>
This is how we could port this procedure to <application>PL/pgSQL</>:
<programlisting>
CREATE OR REPLACE FUNCTION cs_create_job(v_job_id integer) RETURNS void AS $$
DECLARE
a_running_job_count integer;
BEGIN
LOCK TABLE cs_jobs IN EXCLUSIVE MODE;
SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;
IF a_running_job_count &gt; 0 THEN
RAISE EXCEPTION 'Unable to create a new job: a job is currently running';<co id="co.plpgsql-porting-raise">
END IF;
DELETE FROM cs_active_job;
INSERT INTO cs_active_job(job_id) VALUES (v_job_id);
BEGIN
INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
EXCEPTION
WHEN unique_violation THEN <co id="co.plpgsql-porting-exception">
-- don't worry if it already exists
END;
END;
$$ LANGUAGE plpgsql;
</programlisting>
<calloutlist>
<callout arearefs="co.plpgsql-porting-raise">
<para>
The syntax of <literal>RAISE</> is considerably different from
Oracle's statement, although the basic case <literal>RAISE</>
<replaceable class="parameter">exception_name</replaceable> works
similarly.
</para>
</callout>
<callout arearefs="co.plpgsql-porting-exception">
<para>
The exception names supported by <application>PL/pgSQL</> are
different from Oracle's. The set of built-in exception names
is much larger (see <xref linkend="errcodes-appendix">). There
is not currently a way to declare user-defined exception names.
</para>
</callout>
</calloutlist>
The main functional difference between this procedure and the
Oracle equivalent is that the exclusive lock on the <literal>cs_jobs</>
table will be held until the calling transaction completes. Also, if
the caller later aborts (for example due to an error), the effects of
this procedure will be rolled back.
</para>
</example>
</sect2>
<sect2 id="plpgsql-porting-other">
<title>Other Things to Watch For</title>
<para>
This section explains a few other things to watch for when porting
Oracle <application>PL/SQL</> functions to
<productname>PostgreSQL</productname>.
</para>
<sect3 id="plpgsql-porting-exceptions">
<title>Implicit Rollback after Exceptions</title>
<para>
In <application>PL/pgSQL</>, when an exception is caught by an
<literal>EXCEPTION</> clause, all database changes since the block's
<literal>BEGIN</> are automatically rolled back. That is, the behavior
is equivalent to what you'd get in Oracle with:
<programlisting>
BEGIN
SAVEPOINT s1;
... code here ...
EXCEPTION
WHEN ... THEN
ROLLBACK TO s1;
... code here ...
WHEN ... THEN
ROLLBACK TO s1;
... code here ...
END;
</programlisting>
If you are translating an Oracle procedure that uses
<command>SAVEPOINT</> and <command>ROLLBACK TO</> in this style,
your task is easy: just omit the <command>SAVEPOINT</> and
<command>ROLLBACK TO</>. If you have a procedure that uses
<command>SAVEPOINT</> and <command>ROLLBACK TO</> in a different way
then some actual thought will be required.
</para>
</sect3>
<sect3>
<title><command>EXECUTE</command></title>
<para>
The <application>PL/pgSQL</> version of
<command>EXECUTE</command> works similarly to the
<application>PL/SQL</> version, but you have to remember to use
<function>quote_literal</function> and
<function>quote_ident</function> as described in <xref
linkend="plpgsql-statements-executing-dyn">. Constructs of the
type <literal>EXECUTE 'SELECT * FROM $1';</literal> will not work
reliably unless you use these functions.
</para>
</sect3>
<sect3 id="plpgsql-porting-optimization">
<title>Optimizing <application>PL/pgSQL</application> Functions</title>
<para>
<productname>PostgreSQL</> gives you two function creation
modifiers to optimize execution: <quote>volatility</> (whether
the function always returns the same result when given the same
arguments) and <quote>strictness</quote> (whether the function
returns null if any argument is null). Consult the <xref
linkend="sql-createfunction" endterm="sql-createfunction-title">
reference page for details.
</para>
<para>
When making use of these optimization attributes, your
<command>CREATE FUNCTION</command> statement might look something
like this:
<programlisting>
CREATE FUNCTION foo(...) RETURNS integer AS $$
...
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
</programlisting>
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-porting-appendix">
<title>Appendix</title>
<para>
This section contains the code for a set of Oracle-compatible
<function>instr</function> functions that you can use to simplify
your porting efforts.
</para>
<programlisting>
--
-- instr functions that mimic Oracle's counterpart
-- Syntax: instr(string1, string2, [n], [m]) where [] denotes optional parameters.
--
-- Searches string1 beginning at the nth character for the mth occurrence
-- of string2. If n is negative, search backwards. If m is not passed,
-- assume 1 (search starts at first character).
--
CREATE FUNCTION instr(varchar, varchar) RETURNS integer AS $$
DECLARE
pos integer;
BEGIN
pos:= instr($1, $2, 1);
RETURN pos;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
CREATE FUNCTION instr(string varchar, string_to_search varchar, beg_index integer)
RETURNS integer AS $$
DECLARE
pos integer NOT NULL DEFAULT 0;
temp_str varchar;
beg integer;
length integer;
ss_length integer;
BEGIN
IF beg_index &gt; 0 THEN
temp_str := substring(string FROM beg_index);
pos := position(string_to_search IN temp_str);
IF pos = 0 THEN
RETURN 0;
ELSE
RETURN pos + beg_index - 1;
END IF;
ELSE
ss_length := char_length(string_to_search);
length := char_length(string);
beg := length + beg_index - ss_length + 2;
WHILE beg &gt; 0 LOOP
temp_str := substring(string FROM beg FOR ss_length);
pos := position(string_to_search IN temp_str);
IF pos &gt; 0 THEN
RETURN beg;
END IF;
beg := beg - 1;
END LOOP;
RETURN 0;
END IF;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
CREATE FUNCTION instr(string varchar, string_to_search varchar,
beg_index integer, occur_index integer)
RETURNS integer AS $$
DECLARE
pos integer NOT NULL DEFAULT 0;
occur_number integer NOT NULL DEFAULT 0;
temp_str varchar;
beg integer;
i integer;
length integer;
ss_length integer;
BEGIN
IF beg_index &gt; 0 THEN
beg := beg_index;
temp_str := substring(string FROM beg_index);
FOR i IN 1..occur_index LOOP
pos := position(string_to_search IN temp_str);
IF i = 1 THEN
beg := beg + pos - 1;
ELSE
beg := beg + pos;
END IF;
temp_str := substring(string FROM beg + 1);
END LOOP;
IF pos = 0 THEN
RETURN 0;
ELSE
RETURN beg;
END IF;
ELSE
ss_length := char_length(string_to_search);
length := char_length(string);
beg := length + beg_index - ss_length + 2;
WHILE beg &gt; 0 LOOP
temp_str := substring(string FROM beg FOR ss_length);
pos := position(string_to_search IN temp_str);
IF pos &gt; 0 THEN
occur_number := occur_number + 1;
IF occur_number = occur_index THEN
RETURN beg;
END IF;
END IF;
beg := beg - 1;
END LOOP;
RETURN 0;
END IF;
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;
</programlisting>
</sect2>
</sect1>
</chapter>
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