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postgres/doc/src/sgml/xfunc.sgml
Tom Lane 253ff58a1d Improve documentation about function volatility: mention the snapshot 
visibility effects in a couple of places where people are likely to look
for it.  Per discussion of recent question from Karl Nack.
2009-05-27 01:18:06 +00:00

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<!-- $PostgreSQL: pgsql/doc/src/sgml/xfunc.sgml,v 1.138 2009/05/27 01:18:06 tgl Exp $ -->
<sect1 id="xfunc">
<title>User-Defined Functions</title>
<indexterm zone="xfunc">
<primary>function</primary>
<secondary>user-defined</secondary>
</indexterm>
<para>
<productname>PostgreSQL</productname> provides four kinds of
functions:
<itemizedlist>
<listitem>
<para>
query language functions (functions written in
<acronym>SQL</acronym>) (<xref linkend="xfunc-sql">)
</para>
</listitem>
<listitem>
<para>
procedural language functions (functions written in, for
example, <application>PL/pgSQL</> or <application>PL/Tcl</>)
(<xref linkend="xfunc-pl">)
</para>
</listitem>
<listitem>
<para>
internal functions (<xref linkend="xfunc-internal">)
</para>
</listitem>
<listitem>
<para>
C-language functions (<xref linkend="xfunc-c">)
</para>
</listitem>
</itemizedlist>
</para>
<para>
Every kind
of function can take base types, composite types, or
combinations of these as arguments (parameters). In addition,
every kind of function can return a base type or
a composite type. Functions can also be defined to return
sets of base or composite values.
</para>
<para>
Many kinds of functions can take or return certain pseudo-types
(such as polymorphic types), but the available facilities vary.
Consult the description of each kind of function for more details.
</para>
<para>
It's easiest to define <acronym>SQL</acronym>
functions, so we'll start by discussing those.
Most of the concepts presented for <acronym>SQL</acronym> functions
will carry over to the other types of functions.
</para>
<para>
Throughout this chapter, it can be useful to look at the reference
page of the <xref linkend="sql-createfunction"
endterm="sql-createfunction-title"> command to
understand the examples better. Some examples from this chapter
can be found in <filename>funcs.sql</filename> and
<filename>funcs.c</filename> in the <filename>src/tutorial</>
directory in the <productname>PostgreSQL</productname> source
distribution.
</para>
</sect1>
<sect1 id="xfunc-sql">
<title>Query Language (<acronym>SQL</acronym>) Functions</title>
<indexterm zone="xfunc-sql">
<primary>function</primary>
<secondary>user-defined</secondary>
<tertiary>in SQL</tertiary>
</indexterm>
<para>
SQL functions execute an arbitrary list of SQL statements, returning
the result of the last query in the list.
In the simple (non-set)
case, the first row of the last query's result will be returned.
(Bear in mind that <quote>the first row</quote> of a multirow
result is not well-defined unless you use <literal>ORDER BY</>.)
If the last query happens
to return no rows at all, the null value will be returned.
</para>
<para>
Alternatively, an SQL function can be declared to return a set,
by specifying the function's return type as <literal>SETOF
<replaceable>sometype</></literal>, or equivalently by declaring it as
<literal>RETURNS TABLE(<replaceable>columns</>)</literal>. In this case
all rows of the last query's result are returned. Further details appear
below.
</para>
<para>
The body of an SQL function must be a list of SQL
statements separated by semicolons. A semicolon after the last
statement is optional. Unless the function is declared to return
<type>void</>, the last statement must be a <command>SELECT</>,
or an <command>INSERT</>, <command>UPDATE</>, or <command>DELETE</>
that has a <literal>RETURNING</> clause.
</para>
<para>
Any collection of commands in the <acronym>SQL</acronym>
language can be packaged together and defined as a function.
Besides <command>SELECT</command> queries, the commands can include data
modification queries (<command>INSERT</command>,
<command>UPDATE</command>, and <command>DELETE</command>), as well as
other SQL commands. (The only exception is that you cannot put
<command>BEGIN</>, <command>COMMIT</>, <command>ROLLBACK</>, or
<command>SAVEPOINT</> commands into a <acronym>SQL</acronym> function.)
However, the final command
must be a <command>SELECT</command> or have a <literal>RETURNING</>
clause that returns whatever is
specified as the function's return type. Alternatively, if you
want to define a SQL function that performs actions but has no
useful value to return, you can define it as returning <type>void</>.
For example, this function removes rows with negative salaries from
the <literal>emp</> table:
<screen>
CREATE FUNCTION clean_emp() RETURNS void AS '
DELETE FROM emp
WHERE salary &lt; 0;
' LANGUAGE SQL;
SELECT clean_emp();
clean_emp
-----------
(1 row)
</screen>
</para>
<para>
The syntax of the <command>CREATE FUNCTION</command> command requires
the function body to be written as a string constant. It is usually
most convenient to use dollar quoting (see <xref
linkend="sql-syntax-dollar-quoting">) for the string constant.
If you choose to use regular single-quoted string constant syntax,
you must double single quote marks (<literal>'</>) and backslashes
(<literal>\</>) (assuming escape string syntax) in the body of
the function (see <xref linkend="sql-syntax-strings">).
</para>
<para>
Arguments to the SQL function are referenced in the function
body using the syntax <literal>$<replaceable>n</></>: <literal>$1</>
refers to the first argument, <literal>$2</> to the second, and so on.
If an argument is of a composite type, then the dot notation,
e.g., <literal>$1.name</literal>, can be used to access attributes
of the argument. The arguments can only be used as data values,
not as identifiers. Thus for example this is reasonable:
<programlisting>
INSERT INTO mytable VALUES ($1);
</programlisting>
but this will not work:
<programlisting>
INSERT INTO $1 VALUES (42);
</programlisting>
</para>
<sect2 id="xfunc-sql-base-functions">
<title><acronym>SQL</acronym> Functions on Base Types</title>
<para>
The simplest possible <acronym>SQL</acronym> function has no arguments and
simply returns a base type, such as <type>integer</type>:
<screen>
CREATE FUNCTION one() RETURNS integer AS $$
SELECT 1 AS result;
$$ LANGUAGE SQL;
-- Alternative syntax for string literal:
CREATE FUNCTION one() RETURNS integer AS '
SELECT 1 AS result;
' LANGUAGE SQL;
SELECT one();
one
-----
1
</screen>
</para>
<para>
Notice that we defined a column alias within the function body for the result of the function
(with the name <literal>result</>), but this column alias is not visible
outside the function. Hence, the result is labeled <literal>one</>
instead of <literal>result</>.
</para>
<para>
It is almost as easy to define <acronym>SQL</acronym> functions
that take base types as arguments. In the example below, notice
how we refer to the arguments within the function as <literal>$1</>
and <literal>$2</>.
<screen>
CREATE FUNCTION add_em(integer, integer) RETURNS integer AS $$
SELECT $1 + $2;
$$ LANGUAGE SQL;
SELECT add_em(1, 2) AS answer;
answer
--------
3
</screen>
</para>
<para>
Here is a more useful function, which might be used to debit a
bank account:
<programlisting>
CREATE FUNCTION tf1 (integer, numeric) RETURNS integer AS $$
UPDATE bank
SET balance = balance - $2
WHERE accountno = $1;
SELECT 1;
$$ LANGUAGE SQL;
</programlisting>
A user could execute this function to debit account 17 by $100.00 as
follows:
<programlisting>
SELECT tf1(17, 100.0);
</programlisting>
</para>
<para>
In practice one would probably like a more useful result from the
function than a constant 1, so a more likely definition
is:
<programlisting>
CREATE FUNCTION tf1 (integer, numeric) RETURNS numeric AS $$
UPDATE bank
SET balance = balance - $2
WHERE accountno = $1;
SELECT balance FROM bank WHERE accountno = $1;
$$ LANGUAGE SQL;
</programlisting>
which adjusts the balance and returns the new balance.
The same thing could be done in one command using <literal>RETURNING</>:
<programlisting>
CREATE FUNCTION tf1 (integer, numeric) RETURNS numeric AS $$
UPDATE bank
SET balance = balance - $2
WHERE accountno = $1
RETURNING balance;
$$ LANGUAGE SQL;
</programlisting>
</para>
</sect2>
<sect2>
<title><acronym>SQL</acronym> Functions on Composite Types</title>
<para>
When writing functions with arguments of composite
types, we must not only specify which
argument we want (as we did above with <literal>$1</> and <literal>$2</literal>) but
also the desired attribute (field) of that argument. For example,
suppose that
<type>emp</type> is a table containing employee data, and therefore
also the name of the composite type of each row of the table. Here
is a function <function>double_salary</function> that computes what someone's
salary would be if it were doubled:
<screen>
CREATE TABLE emp (
name text,
salary numeric,
age integer,
cubicle point
);
CREATE FUNCTION double_salary(emp) RETURNS numeric AS $$
SELECT $1.salary * 2 AS salary;
$$ LANGUAGE SQL;
SELECT name, double_salary(emp.*) AS dream
FROM emp
WHERE emp.cubicle ~= point '(2,1)';
name | dream
------+-------
Bill | 8400
</screen>
</para>
<para>
Notice the use of the syntax <literal>$1.salary</literal>
to select one field of the argument row value. Also notice
how the calling <command>SELECT</> command uses <literal>*</>
to select
the entire current row of a table as a composite value. The table
row can alternatively be referenced using just the table name,
like this:
<screen>
SELECT name, double_salary(emp) AS dream
FROM emp
WHERE emp.cubicle ~= point '(2,1)';
</screen>
but this usage is deprecated since it's easy to get confused.
</para>
<para>
Sometimes it is handy to construct a composite argument value
on-the-fly. This can be done with the <literal>ROW</> construct.
For example, we could adjust the data being passed to the function:
<screen>
SELECT name, double_salary(ROW(name, salary*1.1, age, cubicle)) AS dream
FROM emp;
</screen>
</para>
<para>
It is also possible to build a function that returns a composite type.
This is an example of a function
that returns a single <type>emp</type> row:
<programlisting>
CREATE FUNCTION new_emp() RETURNS emp AS $$
SELECT text 'None' AS name,
1000.0 AS salary,
25 AS age,
point '(2,2)' AS cubicle;
$$ LANGUAGE SQL;
</programlisting>
In this example we have specified each of the attributes
with a constant value, but any computation
could have been substituted for these constants.
</para>
<para>
Note two important things about defining the function:
<itemizedlist>
<listitem>
<para>
The select list order in the query must be exactly the same as
that in which the columns appear in the table associated
with the composite type. (Naming the columns, as we did above,
is irrelevant to the system.)
</para>
</listitem>
<listitem>
<para>
You must typecast the expressions to match the
definition of the composite type, or you will get errors like this:
<screen>
<computeroutput>
ERROR: function declared to return emp returns varchar instead of text at column 1
</computeroutput>
</screen>
</para>
</listitem>
</itemizedlist>
</para>
<para>
A different way to define the same function is:
<programlisting>
CREATE FUNCTION new_emp() RETURNS emp AS $$
SELECT ROW('None', 1000.0, 25, '(2,2)')::emp;
$$ LANGUAGE SQL;
</programlisting>
Here we wrote a <command>SELECT</> that returns just a single
column of the correct composite type. This isn't really better
in this situation, but it is a handy alternative in some cases
&mdash; for example, if we need to compute the result by calling
another function that returns the desired composite value.
</para>
<para>
We could call this function directly in either of two ways:
<screen>
SELECT new_emp();
new_emp
--------------------------
(None,1000.0,25,"(2,2)")
SELECT * FROM new_emp();
name | salary | age | cubicle
------+--------+-----+---------
None | 1000.0 | 25 | (2,2)
</screen>
The second way is described more fully in <xref
linkend="xfunc-sql-table-functions">.
</para>
<para>
When you use a function that returns a composite type,
you might want only one field (attribute) from its result.
You can do that with syntax like this:
<screen>
SELECT (new_emp()).name;
name
------
None
</screen>
The extra parentheses are needed to keep the parser from getting
confused. If you try to do it without them, you get something like this:
<screen>
SELECT new_emp().name;
ERROR: syntax error at or near "."
LINE 1: SELECT new_emp().name;
^
</screen>
</para>
<para>
Another option is to use
functional notation for extracting an attribute. The simple way
to explain this is that we can use the
notations <literal>attribute(table)</> and <literal>table.attribute</>
interchangeably.
<screen>
SELECT name(new_emp());
name
------
None
</screen>
<screen>
-- This is the same as:
-- SELECT emp.name AS youngster FROM emp WHERE emp.age &lt; 30;
SELECT name(emp) AS youngster FROM emp WHERE age(emp) &lt; 30;
youngster
-----------
Sam
Andy
</screen>
</para>
<tip>
<para>
The equivalence between functional notation and attribute notation
makes it possible to use functions on composite types to emulate
<quote>computed fields</>.
<indexterm>
<primary>computed field</primary>
</indexterm>
<indexterm>
<primary>field</primary>
<secondary>computed</secondary>
</indexterm>
For example, using the previous definition
for <literal>double_salary(emp)</>, we can write
<screen>
SELECT emp.name, emp.double_salary FROM emp;
</screen>
An application using this wouldn't need to be directly aware that
<literal>double_salary</> isn't a real column of the table.
(You can also emulate computed fields with views.)
</para>
</tip>
<para>
Another way to use a function returning a composite type is to pass the
result to another function that accepts the correct row type as input:
<screen>
CREATE FUNCTION getname(emp) RETURNS text AS $$
SELECT $1.name;
$$ LANGUAGE SQL;
SELECT getname(new_emp());
getname
---------
None
(1 row)
</screen>
</para>
<para>
Still another way to use a function that returns a composite type is to
call it as a table function, as described in <xref
linkend="xfunc-sql-table-functions">.
</para>
</sect2>
<sect2 id="xfunc-output-parameters">
<title><acronym>SQL</> Functions with Output Parameters</title>
<indexterm>
<primary>function</primary>
<secondary>output parameter</secondary>
</indexterm>
<para>
An alternative way of describing a function's results is to define it
with <firstterm>output parameters</>, as in this example:
<screen>
CREATE FUNCTION add_em (IN x int, IN y int, OUT sum int)
AS 'SELECT $1 + $2'
LANGUAGE SQL;
SELECT add_em(3,7);
add_em
--------
10
(1 row)
</screen>
This is not essentially different from the version of <literal>add_em</>
shown in <xref linkend="xfunc-sql-base-functions">. The real value of
output parameters is that they provide a convenient way of defining
functions that return several columns. For example,
<screen>
CREATE FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int)
AS 'SELECT $1 + $2, $1 * $2'
LANGUAGE SQL;
SELECT * FROM sum_n_product(11,42);
sum | product
-----+---------
53 | 462
(1 row)
</screen>
What has essentially happened here is that we have created an anonymous
composite type for the result of the function. The above example has
the same end result as
<screen>
CREATE TYPE sum_prod AS (sum int, product int);
CREATE FUNCTION sum_n_product (int, int) RETURNS sum_prod
AS 'SELECT $1 + $2, $1 * $2'
LANGUAGE SQL;
</screen>
but not having to bother with the separate composite type definition
is often handy.
</para>
<para>
Notice that output parameters are not included in the calling argument
list when invoking such a function from SQL. This is because
<productname>PostgreSQL</productname> considers only the input
parameters to define the function's calling signature. That means
also that only the input parameters matter when referencing the function
for purposes such as dropping it. We could drop the above function
with either of
<screen>
DROP FUNCTION sum_n_product (x int, y int, OUT sum int, OUT product int);
DROP FUNCTION sum_n_product (int, int);
</screen>
</para>
<para>
Parameters can be marked as <literal>IN</> (the default),
<literal>OUT</>, <literal>INOUT</>, or <literal>VARIADIC</>.
An <literal>INOUT</>
parameter serves as both an input parameter (part of the calling
argument list) and an output parameter (part of the result record type).
<literal>VARIADIC</> parameters are input parameters, but are treated
specially as described next.
</para>
</sect2>
<sect2 id="xfunc-sql-variadic-functions">
<title><acronym>SQL</> Functions with Variable Numbers of Arguments</title>
<indexterm>
<primary>function</primary>
<secondary>variadic</secondary>
</indexterm>
<indexterm>
<primary>variadic function</primary>
</indexterm>
<para>
<acronym>SQL</acronym> functions can be declared to accept
variable numbers of arguments, so long as all the <quote>optional</>
arguments are of the same data type. The optional arguments will be
passed to the function as an array. The function is declared by
marking the last parameter as <literal>VARIADIC</>; this parameter
must be declared as being of an array type. For example:
<screen>
CREATE FUNCTION mleast(VARIADIC numeric[]) RETURNS numeric AS $$
SELECT min($1[i]) FROM generate_subscripts($1, 1) g(i);
$$ LANGUAGE SQL;
SELECT mleast(10, -1, 5, 4.4);
mleast
--------
-1
(1 row)
</screen>
Effectively, all the actual arguments at or beyond the
<literal>VARIADIC</> position are gathered up into a one-dimensional
array, as if you had written
<screen>
SELECT mleast(ARRAY[10, -1, 5, 4.4]); -- doesn't work
</screen>
You can't actually write that, though &mdash; or at least, it will
not match this function definition. A parameter marked
<literal>VARIADIC</> matches one or more occurrences of its element
type, not of its own type.
</para>
<para>
Sometimes it is useful to be able to pass an already-constructed array
to a variadic function; this is particularly handy when one variadic
function wants to pass on its array parameter to another one. You can
do that by specifying <literal>VARIADIC</> in the call:
<screen>
SELECT mleast(VARIADIC ARRAY[10, -1, 5, 4.4]);
</screen>
This prevents expansion of the function's variadic parameter into its
element type, thereby allowing the array argument value to match
normally. <literal>VARIADIC</> can only be attached to the last
actual argument of a function call.
</para>
</sect2>
<sect2 id="xfunc-sql-parameter-defaults">
<title><acronym>SQL</> Functions with Default Values for Arguments</title>
<indexterm>
<primary>function</primary>
<secondary>default values for arguments</secondary>
</indexterm>
<para>
Functions can be declared with default values for some or all input
arguments. The default values are inserted whenever the function is
called with insufficiently many actual arguments. Since arguments
can only be omitted from the end of the actual argument list, all
parameters after a parameter with a default value have to have
default values as well.
</para>
<para>
For example:
<screen>
CREATE FUNCTION foo(a int, b int DEFAULT 2, c int DEFAULT 3)
RETURNS int
LANGUAGE SQL
AS $$
SELECT $1 + $2 + $3;
$$;
SELECT foo(10, 20, 30);
foo
-----
60
(1 row)
SELECT foo(10, 20);
foo
-----
33
(1 row)
SELECT foo(10);
foo
-----
15
(1 row)
SELECT foo(); -- fails since there is no default for the first argument
ERROR: function foo() does not exist
</screen>
The <literal>=</literal> sign can also be used in place of the
key word <literal>DEFAULT</literal>,
</para>
</sect2>
<sect2 id="xfunc-sql-table-functions">
<title><acronym>SQL</acronym> Functions as Table Sources</title>
<para>
All SQL functions can be used in the <literal>FROM</> clause of a query,
but it is particularly useful for functions returning composite types.
If the function is defined to return a base type, the table function
produces a one-column table. If the function is defined to return
a composite type, the table function produces a column for each attribute
of the composite type.
</para>
<para>
Here is an example:
<screen>
CREATE TABLE foo (fooid int, foosubid int, fooname text);
INSERT INTO foo VALUES (1, 1, 'Joe');
INSERT INTO foo VALUES (1, 2, 'Ed');
INSERT INTO foo VALUES (2, 1, 'Mary');
CREATE FUNCTION getfoo(int) RETURNS foo AS $$
SELECT * FROM foo WHERE fooid = $1;
$$ LANGUAGE SQL;
SELECT *, upper(fooname) FROM getfoo(1) AS t1;
fooid | foosubid | fooname | upper
-------+----------+---------+-------
1 | 1 | Joe | JOE
(1 row)
</screen>
As the example shows, we can work with the columns of the function's
result just the same as if they were columns of a regular table.
</para>
<para>
Note that we only got one row out of the function. This is because
we did not use <literal>SETOF</>. That is described in the next section.
</para>
</sect2>
<sect2 id="xfunc-sql-functions-returning-set">
<title><acronym>SQL</acronym> Functions Returning Sets</title>
<indexterm>
<primary>function</primary>
<secondary>with SETOF</secondary>
</indexterm>
<para>
When an SQL function is declared as returning <literal>SETOF
<replaceable>sometype</></literal>, the function's final
query is executed to completion, and each row it
outputs is returned as an element of the result set.
</para>
<para>
This feature is normally used when calling the function in the <literal>FROM</>
clause. In this case each row returned by the function becomes
a row of the table seen by the query. For example, assume that
table <literal>foo</> has the same contents as above, and we say:
<programlisting>
CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS $$
SELECT * FROM foo WHERE fooid = $1;
$$ LANGUAGE SQL;
SELECT * FROM getfoo(1) AS t1;
</programlisting>
Then we would get:
<screen>
fooid | foosubid | fooname
-------+----------+---------
1 | 1 | Joe
1 | 2 | Ed
(2 rows)
</screen>
</para>
<para>
It is also possible to return multiple rows with the columns defined by
output parameters, like this:
<programlisting>
CREATE FUNCTION sum_n_product_with_tab (x int, OUT sum int, OUT product int) RETURNS SETOF record AS $$
SELECT $1 + tab.y, $1 * tab.y FROM tab;
$$ LANGUAGE SQL;
</programlisting>
The key point here is that you must write <literal>RETURNS SETOF record</>
to indicate that the function returns multiple rows instead of just one.
If there is only one output parameter, write that parameter's type
instead of <type>record</>.
</para>
<para>
Currently, functions returning sets can also be called in the select list
of a query. For each row that the query
generates by itself, the function returning set is invoked, and an output
row is generated for each element of the function's result set. Note,
however, that this capability is deprecated and might be removed in future
releases. The following is an example function returning a set from the
select list:
<screen>
CREATE FUNCTION listchildren(text) RETURNS SETOF text AS $$
SELECT name FROM nodes WHERE parent = $1
$$ LANGUAGE SQL;
SELECT * FROM nodes;
name | parent
-----------+--------
Top |
Child1 | Top
Child2 | Top
Child3 | Top
SubChild1 | Child1
SubChild2 | Child1
(6 rows)
SELECT listchildren('Top');
listchildren
--------------
Child1
Child2
Child3
(3 rows)
SELECT name, listchildren(name) FROM nodes;
name | listchildren
--------+--------------
Top | Child1
Top | Child2
Top | Child3
Child1 | SubChild1
Child1 | SubChild2
(5 rows)
</screen>
In the last <command>SELECT</command>,
notice that no output row appears for <literal>Child2</>, <literal>Child3</>, etc.
This happens because <function>listchildren</function> returns an empty set
for those arguments, so no result rows are generated.
</para>
<note>
<para>
If a function's last command is <command>INSERT</>, <command>UPDATE</>,
or <command>DELETE</> with <literal>RETURNING</>, that command will
always be executed to completion, even if the function is not declared
with <literal>SETOF</> or the calling query does not fetch all the
result rows. Any extra rows produced by the <literal>RETURNING</>
clause are silently dropped, but the commanded table modifications
still happen (and are all completed before returning from the function).
</para>
</note>
</sect2>
<sect2 id="xfunc-sql-functions-returning-table">
<title><acronym>SQL</acronym> Functions Returning <literal>TABLE</></title>
<indexterm>
<primary>function</primary>
<secondary>RETURNS TABLE</secondary>
</indexterm>
<para>
There is another way to declare a function as returning a set,
which is to use the syntax
<literal>RETURNS TABLE(<replaceable>columns</>)</literal>.
This is equivalent to using one or more <literal>OUT</> parameters plus
marking the function as returning <literal>SETOF record</> (or
<literal>SETOF</> a single output parameter's type, as appropriate).
This notation is specified in recent versions of the SQL standard, and
thus may be more portable than using <literal>SETOF</>.
</para>
<para>
For example, the preceding sum-and-product example could also be
done this way:
<programlisting>
CREATE FUNCTION sum_n_product_with_tab (x int) RETURNS TABLE(sum int, product int) AS $$
SELECT $1 + tab.y, $1 * tab.y FROM tab;
$$ LANGUAGE SQL;
</programlisting>
It is not allowed to use explicit <literal>OUT</> or <literal>INOUT</>
parameters with the <literal>RETURNS TABLE</> notation &mdash; you must
put all the output columns in the <literal>TABLE</> list.
</para>
</sect2>
<sect2>
<title>Polymorphic <acronym>SQL</acronym> Functions</title>
<para>
<acronym>SQL</acronym> functions can be declared to accept and
return the polymorphic types <type>anyelement</type>,
<type>anyarray</type>, <type>anynonarray</type>, and
<type>anyenum</type>. See <xref
linkend="extend-types-polymorphic"> for a more detailed
explanation of polymorphic functions. Here is a polymorphic
function <function>make_array</function> that builds up an array
from two arbitrary data type elements:
<screen>
CREATE FUNCTION make_array(anyelement, anyelement) RETURNS anyarray AS $$
SELECT ARRAY[$1, $2];
$$ LANGUAGE SQL;
SELECT make_array(1, 2) AS intarray, make_array('a'::text, 'b') AS textarray;
intarray | textarray
----------+-----------
{1,2} | {a,b}
(1 row)
</screen>
</para>
<para>
Notice the use of the typecast <literal>'a'::text</literal>
to specify that the argument is of type <type>text</type>. This is
required if the argument is just a string literal, since otherwise
it would be treated as type
<type>unknown</type>, and array of <type>unknown</type> is not a valid
type.
Without the typecast, you will get errors like this:
<screen>
<computeroutput>
ERROR: could not determine polymorphic type because input has type "unknown"
</computeroutput>
</screen>
</para>
<para>
It is permitted to have polymorphic arguments with a fixed
return type, but the converse is not. For example:
<screen>
CREATE FUNCTION is_greater(anyelement, anyelement) RETURNS boolean AS $$
SELECT $1 &gt; $2;
$$ LANGUAGE SQL;
SELECT is_greater(1, 2);
is_greater
------------
f
(1 row)
CREATE FUNCTION invalid_func() RETURNS anyelement AS $$
SELECT 1;
$$ LANGUAGE SQL;
ERROR: cannot determine result data type
DETAIL: A function returning a polymorphic type must have at least one polymorphic argument.
</screen>
</para>
<para>
Polymorphism can be used with functions that have output arguments.
For example:
<screen>
CREATE FUNCTION dup (f1 anyelement, OUT f2 anyelement, OUT f3 anyarray)
AS 'select $1, array[$1,$1]' LANGUAGE SQL;
SELECT * FROM dup(22);
f2 | f3
----+---------
22 | {22,22}
(1 row)
</screen>
</para>
<para>
Polymorphism can also be used with variadic functions.
For example:
<screen>
CREATE FUNCTION anyleast (VARIADIC anyarray) RETURNS anyelement AS $$
SELECT min($1[i]) FROM generate_subscripts($1, 1) g(i);
$$ LANGUAGE SQL;
SELECT anyleast(10, -1, 5, 4);
anyleast
----------
-1
(1 row)
SELECT anyleast('abc'::text, 'def');
anyleast
----------
abc
(1 row)
CREATE FUNCTION concat(text, VARIADIC anyarray) RETURNS text AS $$
SELECT array_to_string($2, $1);
$$ LANGUAGE SQL;
SELECT concat('|', 1, 4, 2);
concat
--------
1|4|2
(1 row)
</screen>
</para>
</sect2>
</sect1>
<sect1 id="xfunc-overload">
<title>Function Overloading</title>
<indexterm zone="xfunc-overload">
<primary>overloading</primary>
<secondary>functions</secondary>
</indexterm>
<para>
More than one function can be defined with the same SQL name, so long
as the arguments they take are different. In other words,
function names can be <firstterm>overloaded</firstterm>. When a
query is executed, the server will determine which function to
call from the data types and the number of the provided arguments.
Overloading can also be used to simulate functions with a variable
number of arguments, up to a finite maximum number.
</para>
<para>
When creating a family of overloaded functions, one should be
careful not to create ambiguities. For instance, given the
functions:
<programlisting>
CREATE FUNCTION test(int, real) RETURNS ...
CREATE FUNCTION test(smallint, double precision) RETURNS ...
</programlisting>
it is not immediately clear which function would be called with
some trivial input like <literal>test(1, 1.5)</literal>. The
currently implemented resolution rules are described in
<xref linkend="typeconv">, but it is unwise to design a system that subtly
relies on this behavior.
</para>
<para>
A function that takes a single argument of a composite type should
generally not have the same name as any attribute (field) of that type.
Recall that <literal>attribute(table)</literal> is considered equivalent
to <literal>table.attribute</literal>. In the case that there is an
ambiguity between a function on a composite type and an attribute of
the composite type, the attribute will always be used. It is possible
to override that choice by schema-qualifying the function name
(that is, <literal>schema.func(table)</literal>) but it's better to
avoid the problem by not choosing conflicting names.
</para>
<para>
Another possible conflict is between variadic and non-variadic functions.
For instance, it is possible to create both <literal>foo(numeric)</> and
<literal>foo(VARIADIC numeric[])</>. In this case it is unclear which one
should be matched to a call providing a single numeric argument, such as
<literal>foo(10.1)</>. The rule is that the function appearing
earlier in the search path is used, or if the two functions are in the
same schema, the non-variadic one is preferred.
</para>
<para>
When overloading C-language functions, there is an additional
constraint: The C name of each function in the family of
overloaded functions must be different from the C names of all
other functions, either internal or dynamically loaded. If this
rule is violated, the behavior is not portable. You might get a
run-time linker error, or one of the functions will get called
(usually the internal one). The alternative form of the
<literal>AS</> clause for the SQL <command>CREATE
FUNCTION</command> command decouples the SQL function name from
the function name in the C source code. For instance:
<programlisting>
CREATE FUNCTION test(int) RETURNS int
AS '<replaceable>filename</>', 'test_1arg'
LANGUAGE C;
CREATE FUNCTION test(int, int) RETURNS int
AS '<replaceable>filename</>', 'test_2arg'
LANGUAGE C;
</programlisting>
The names of the C functions here reflect one of many possible conventions.
</para>
</sect1>
<sect1 id="xfunc-volatility">
<title>Function Volatility Categories</title>
<indexterm zone="xfunc-volatility">
<primary>volatility</primary>
<secondary>functions</secondary>
</indexterm>
<indexterm zone="xfunc-volatility">
<primary>VOLATILE</primary>
</indexterm>
<indexterm zone="xfunc-volatility">
<primary>STABLE</primary>
</indexterm>
<indexterm zone="xfunc-volatility">
<primary>IMMUTABLE</primary>
</indexterm>
<para>
Every function has a <firstterm>volatility</> classification, with
the possibilities being <literal>VOLATILE</>, <literal>STABLE</>, or
<literal>IMMUTABLE</>. <literal>VOLATILE</> is the default if the
<xref linkend="sql-createfunction" endterm="sql-createfunction-title">
command does not specify a category. The volatility category is a
promise to the optimizer about the behavior of the function:
<itemizedlist>
<listitem>
<para>
A <literal>VOLATILE</> function can do anything, including modifying
the database. It can return different results on successive calls with
the same arguments. The optimizer makes no assumptions about the
behavior of such functions. A query using a volatile function will
re-evaluate the function at every row where its value is needed.
</para>
</listitem>
<listitem>
<para>
A <literal>STABLE</> function cannot modify the database and is
guaranteed to return the same results given the same arguments
for all rows within a single statement. This category allows the
optimizer to optimize multiple calls of the function to a single
call. In particular, it is safe to use an expression containing
such a function in an index scan condition. (Since an index scan
will evaluate the comparison value only once, not once at each
row, it is not valid to use a <literal>VOLATILE</> function in an
index scan condition.)
</para>
</listitem>
<listitem>
<para>
An <literal>IMMUTABLE</> function cannot modify the database and is
guaranteed to return the same results given the same arguments forever.
This category allows the optimizer to pre-evaluate the function when
a query calls it with constant arguments. For example, a query like
<literal>SELECT ... WHERE x = 2 + 2</> can be simplified on sight to
<literal>SELECT ... WHERE x = 4</>, because the function underlying
the integer addition operator is marked <literal>IMMUTABLE</>.
</para>
</listitem>
</itemizedlist>
</para>
<para>
For best optimization results, you should label your functions with the
strictest volatility category that is valid for them.
</para>
<para>
Any function with side-effects <emphasis>must</> be labeled
<literal>VOLATILE</>, so that calls to it cannot be optimized away.
Even a function with no side-effects needs to be labeled
<literal>VOLATILE</> if its value can change within a single query;
some examples are <literal>random()</>, <literal>currval()</>,
<literal>timeofday()</>.
</para>
<para>
Another important example is that the <function>current_timestamp</>
family of functions qualify as <literal>STABLE</>, since their values do
not change within a transaction.
</para>
<para>
There is relatively little difference between <literal>STABLE</> and
<literal>IMMUTABLE</> categories when considering simple interactive
queries that are planned and immediately executed: it doesn't matter
a lot whether a function is executed once during planning or once during
query execution startup. But there is a big difference if the plan is
saved and reused later. Labeling a function <literal>IMMUTABLE</> when
it really isn't might allow it to be prematurely folded to a constant during
planning, resulting in a stale value being re-used during subsequent uses
of the plan. This is a hazard when using prepared statements or when
using function languages that cache plans (such as
<application>PL/pgSQL</>).
</para>
<para>
For functions written in SQL or in any of the standard procedural
languages, there is a second important property determined by the
volatility category, namely the visibility of any data changes that have
been made by the SQL command that is calling the function. A
<literal>VOLATILE</> function will see such changes, a <literal>STABLE</>
or <literal>IMMUTABLE</> function will not. This behavior is implemented
using the snapshotting behavior of MVCC (see <xref linkend="mvcc">):
<literal>STABLE</> and <literal>IMMUTABLE</> functions use a snapshot
established as of the start of the calling query, whereas
<literal>VOLATILE</> functions obtain a fresh snapshot at the start of
each query they execute.
</para>
<note>
<para>
Functions written in C can manage snapshots however they want, but it's
usually a good idea to make C functions work this way too.
</para>
</note>
<para>
Because of this snapshotting behavior,
a function containing only <command>SELECT</> commands can safely be
marked <literal>STABLE</>, even if it selects from tables that might be
undergoing modifications by concurrent queries.
<productname>PostgreSQL</productname> will execute all commands of a
<literal>STABLE</> function using the snapshot established for the
calling query, and so it will see a fixed view of the database throughout
that query.
</para>
<para>
The same snapshotting behavior is used for <command>SELECT</> commands
within <literal>IMMUTABLE</> functions. It is generally unwise to select
from database tables within an <literal>IMMUTABLE</> function at all,
since the immutability will be broken if the table contents ever change.
However, <productname>PostgreSQL</productname> does not enforce that you
do not do that.
</para>
<para>
A common error is to label a function <literal>IMMUTABLE</> when its
results depend on a configuration parameter. For example, a function
that manipulates timestamps might well have results that depend on the
<xref linkend="guc-timezone"> setting. For safety, such functions should
be labeled <literal>STABLE</> instead.
</para>
<note>
<para>
Before <productname>PostgreSQL</productname> release 8.0, the requirement
that <literal>STABLE</> and <literal>IMMUTABLE</> functions cannot modify
the database was not enforced by the system. Releases 8.0 and later enforce it
by requiring SQL functions and procedural language functions of these
categories to contain no SQL commands other than <command>SELECT</>.
(This is not a completely bulletproof test, since such functions could
still call <literal>VOLATILE</> functions that modify the database.
If you do that, you will find that the <literal>STABLE</> or
<literal>IMMUTABLE</> function does not notice the database changes
applied by the called function, since they are hidden from its snapshot.)
</para>
</note>
</sect1>
<sect1 id="xfunc-pl">
<title>Procedural Language Functions</title>
<para>
<productname>PostgreSQL</productname> allows user-defined functions
to be written in other languages besides SQL and C. These other
languages are generically called <firstterm>procedural
languages</firstterm> (<acronym>PL</>s).
Procedural languages aren't built into the
<productname>PostgreSQL</productname> server; they are offered
by loadable modules.
See <xref linkend="xplang"> and following chapters for more
information.
</para>
</sect1>
<sect1 id="xfunc-internal">
<title>Internal Functions</title>
<indexterm zone="xfunc-internal"><primary>function</><secondary>internal</></>
<para>
Internal functions are functions written in C that have been statically
linked into the <productname>PostgreSQL</productname> server.
The <quote>body</quote> of the function definition
specifies the C-language name of the function, which need not be the
same as the name being declared for SQL use.
(For reasons of backwards compatibility, an empty body
is accepted as meaning that the C-language function name is the
same as the SQL name.)
</para>
<para>
Normally, all internal functions present in the
server are declared during the initialization of the database cluster (<command>initdb</command>),
but a user could use <command>CREATE FUNCTION</command>
to create additional alias names for an internal function.
Internal functions are declared in <command>CREATE FUNCTION</command>
with language name <literal>internal</literal>. For instance, to
create an alias for the <function>sqrt</function> function:
<programlisting>
CREATE FUNCTION square_root(double precision) RETURNS double precision
AS 'dsqrt'
LANGUAGE internal
STRICT;
</programlisting>
(Most internal functions expect to be declared <quote>strict</quote>.)
</para>
<note>
<para>
Not all <quote>predefined</quote> functions are
<quote>internal</quote> in the above sense. Some predefined
functions are written in SQL.
</para>
</note>
</sect1>
<sect1 id="xfunc-c">
<title>C-Language Functions</title>
<indexterm zone="xfunc-c">
<primary>function</primary>
<secondary>user-defined</secondary>
<tertiary>in C</tertiary>
</indexterm>
<para>
User-defined functions can be written in C (or a language that can
be made compatible with C, such as C++). Such functions are
compiled into dynamically loadable objects (also called shared
libraries) and are loaded by the server on demand. The dynamic
loading feature is what distinguishes <quote>C language</> functions
from <quote>internal</> functions &mdash; the actual coding conventions
are essentially the same for both. (Hence, the standard internal
function library is a rich source of coding examples for user-defined
C functions.)
</para>
<para>
Two different calling conventions are currently used for C functions.
The newer <quote>version 1</quote> calling convention is indicated by writing
a <literal>PG_FUNCTION_INFO_V1()</literal> macro call for the function,
as illustrated below. Lack of such a macro indicates an old-style
(<quote>version 0</quote>) function. The language name specified in <command>CREATE FUNCTION</command>
is <literal>C</literal> in either case. Old-style functions are now deprecated
because of portability problems and lack of functionality, but they
are still supported for compatibility reasons.
</para>
<sect2 id="xfunc-c-dynload">
<title>Dynamic Loading</title>
<indexterm zone="xfunc-c-dynload">
<primary>dynamic loading</primary>
</indexterm>
<para>
The first time a user-defined function in a particular
loadable object file is called in a session,
the dynamic loader loads that object file into memory so that the
function can be called. The <command>CREATE FUNCTION</command>
for a user-defined C function must therefore specify two pieces of
information for the function: the name of the loadable
object file, and the C name (link symbol) of the specific function to call
within that object file. If the C name is not explicitly specified then
it is assumed to be the same as the SQL function name.
</para>
<para>
The following algorithm is used to locate the shared object file
based on the name given in the <command>CREATE FUNCTION</command>
command:
<orderedlist>
<listitem>
<para>
If the name is an absolute path, the given file is loaded.
</para>
</listitem>
<listitem>
<para>
If the name starts with the string <literal>$libdir</literal>,
that part is replaced by the <productname>PostgreSQL</> package
library directory
name, which is determined at build time.<indexterm><primary>$libdir</></>
</para>
</listitem>
<listitem>
<para>
If the name does not contain a directory part, the file is
searched for in the path specified by the configuration variable
<xref linkend="guc-dynamic-library-path">.<indexterm><primary>dynamic_library_path</></>
</para>
</listitem>
<listitem>
<para>
Otherwise (the file was not found in the path, or it contains a
non-absolute directory part), the dynamic loader will try to
take the name as given, which will most likely fail. (It is
unreliable to depend on the current working directory.)
</para>
</listitem>
</orderedlist>
If this sequence does not work, the platform-specific shared
library file name extension (often <filename>.so</filename>) is
appended to the given name and this sequence is tried again. If
that fails as well, the load will fail.
</para>
<para>
It is recommended to locate shared libraries either relative to
<literal>$libdir</literal> or through the dynamic library path.
This simplifies version upgrades if the new installation is at a
different location. The actual directory that
<literal>$libdir</literal> stands for can be found out with the
command <literal>pg_config --pkglibdir</literal>.
</para>
<para>
The user ID the <productname>PostgreSQL</productname> server runs
as must be able to traverse the path to the file you intend to
load. Making the file or a higher-level directory not readable
and/or not executable by the <systemitem>postgres</systemitem>
user is a common mistake.
</para>
<para>
In any case, the file name that is given in the
<command>CREATE FUNCTION</command> command is recorded literally
in the system catalogs, so if the file needs to be loaded again
the same procedure is applied.
</para>
<note>
<para>
<productname>PostgreSQL</productname> will not compile a C function
automatically. The object file must be compiled before it is referenced
in a <command>CREATE
FUNCTION</> command. See <xref linkend="dfunc"> for additional
information.
</para>
</note>
<indexterm zone="xfunc-c-dynload">
<primary>magic block</primary>
</indexterm>
<para>
To ensure that a dynamically loaded object file is not loaded into an
incompatible server, <productname>PostgreSQL</productname> checks that the
file contains a <quote>magic block</> with the appropriate contents.
This allows the server to detect obvious incompatibilities, such as code
compiled for a different major version of
<productname>PostgreSQL</productname>. A magic block is required as of
<productname>PostgreSQL</productname> 8.2. To include a magic block,
write this in one (and only one) of the module source files, after having
included the header <filename>fmgr.h</>:
<programlisting>
#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif
</programlisting>
The <literal>#ifdef</> test can be omitted if the code doesn't
need to compile against pre-8.2 <productname>PostgreSQL</productname>
releases.
</para>
<para>
After it is used for the first time, a dynamically loaded object
file is retained in memory. Future calls in the same session to
the function(s) in that file will only incur the small overhead of
a symbol table lookup. If you need to force a reload of an object
file, for example after recompiling it, use the <xref
linkend="sql-load" endterm="sql-load-title"> command or begin a
fresh session.
</para>
<indexterm zone="xfunc-c-dynload">
<primary>_PG_init</primary>
</indexterm>
<indexterm zone="xfunc-c-dynload">
<primary>_PG_fini</primary>
</indexterm>
<indexterm zone="xfunc-c-dynload">
<primary>library initialization function</primary>
</indexterm>
<indexterm zone="xfunc-c-dynload">
<primary>library finalization function</primary>
</indexterm>
<para>
Optionally, a dynamically loaded file can contain initialization and
finalization functions. If the file includes a function named
<function>_PG_init</>, that function will be called immediately after
loading the file. The function receives no parameters and should
return void. If the file includes a function named
<function>_PG_fini</>, that function will be called immediately before
unloading the file. Likewise, the function receives no parameters and
should return void. Note that <function>_PG_fini</> will only be called
during an unload of the file, not during process termination.
(Presently, an unload only happens in the context of re-loading
the file due to an explicit <command>LOAD</> command.)
</para>
</sect2>
<sect2 id="xfunc-c-basetype">
<title>Base Types in C-Language Functions</title>
<indexterm zone="xfunc-c-basetype">
<primary>data type</primary>
<secondary>internal organization</secondary>
</indexterm>
<para>
To know how to write C-language functions, you need to know how
<productname>PostgreSQL</productname> internally represents base
data types and how they can be passed to and from functions.
Internally, <productname>PostgreSQL</productname> regards a base
type as a <quote>blob of memory</quote>. The user-defined
functions that you define over a type in turn define the way that
<productname>PostgreSQL</productname> can operate on it. That
is, <productname>PostgreSQL</productname> will only store and
retrieve the data from disk and use your user-defined functions
to input, process, and output the data.
</para>
<para>
Base types can have one of three internal formats:
<itemizedlist>
<listitem>
<para>
pass by value, fixed-length
</para>
</listitem>
<listitem>
<para>
pass by reference, fixed-length
</para>
</listitem>
<listitem>
<para>
pass by reference, variable-length
</para>
</listitem>
</itemizedlist>
</para>
<para>
By-value types can only be 1, 2, or 4 bytes in length
(also 8 bytes, if <literal>sizeof(Datum)</literal> is 8 on your machine).
You should be careful to define your types such that they will be the
same size (in bytes) on all architectures. For example, the
<literal>long</literal> type is dangerous because it is 4 bytes on some
machines and 8 bytes on others, whereas <type>int</type> type is 4 bytes
on most Unix machines. A reasonable implementation of the
<type>int4</type> type on Unix machines might be:
<programlisting>
/* 4-byte integer, passed by value */
typedef int int4;
</programlisting>
</para>
<para>
On the other hand, fixed-length types of any size can
be passed by-reference. For example, here is a sample
implementation of a <productname>PostgreSQL</productname> type:
<programlisting>
/* 16-byte structure, passed by reference */
typedef struct
{
double x, y;
} Point;
</programlisting>
Only pointers to such types can be used when passing
them in and out of <productname>PostgreSQL</productname> functions.
To return a value of such a type, allocate the right amount of
memory with <literal>palloc</literal>, fill in the allocated memory,
and return a pointer to it. (Also, if you just want to return the
same value as one of your input arguments that's of the same data type,
you can skip the extra <literal>palloc</literal> and just return the
pointer to the input value.)
</para>
<para>
Finally, all variable-length types must also be passed
by reference. All variable-length types must begin
with a length field of exactly 4 bytes, and all data to
be stored within that type must be located in the memory
immediately following that length field. The
length field contains the total length of the structure,
that is, it includes the size of the length field
itself.
</para>
<warning>
<para>
<emphasis>Never</> modify the contents of a pass-by-reference input
value. If you do so you are likely to corrupt on-disk data, since
the pointer you are given might point directly into a disk buffer.
The sole exception to this rule is explained in
<xref linkend="xaggr">.
</para>
</warning>
<para>
As an example, we can define the type <type>text</type> as
follows:
<programlisting>
typedef struct {
int4 length;
char data[1];
} text;
</programlisting>
Obviously, the data field declared here is not long enough to hold
all possible strings. Since it's impossible to declare a variable-size
structure in <acronym>C</acronym>, we rely on the knowledge that the
<acronym>C</acronym> compiler won't range-check array subscripts. We
just allocate the necessary amount of space and then access the array as
if it were declared the right length. (This is a common trick, which
you can read about in many textbooks about C.)
</para>
<para>
When manipulating
variable-length types, we must be careful to allocate
the correct amount of memory and set the length field correctly.
For example, if we wanted to store 40 bytes in a <structname>text</>
structure, we might use a code fragment like this:
<programlisting><![CDATA[
#include "postgres.h"
...
char buffer[40]; /* our source data */
...
text *destination = (text *) palloc(VARHDRSZ + 40);
destination->length = VARHDRSZ + 40;
memcpy(destination->data, buffer, 40);
...
]]>
</programlisting>
<literal>VARHDRSZ</> is the same as <literal>sizeof(int4)</>, but
it's considered good style to use the macro <literal>VARHDRSZ</>
to refer to the size of the overhead for a variable-length type.
</para>
<para>
<xref linkend="xfunc-c-type-table"> specifies which C type
corresponds to which SQL type when writing a C-language function
that uses a built-in type of <productname>PostgreSQL</>.
The <quote>Defined In</quote> column gives the header file that
needs to be included to get the type definition. (The actual
definition might be in a different file that is included by the
listed file. It is recommended that users stick to the defined
interface.) Note that you should always include
<filename>postgres.h</filename> first in any source file, because
it declares a number of things that you will need anyway.
</para>
<table tocentry="1" id="xfunc-c-type-table">
<title>Equivalent C Types for Built-In SQL Types</title>
<tgroup cols="3">
<thead>
<row>
<entry>
SQL Type
</entry>
<entry>
C Type
</entry>
<entry>
Defined In
</entry>
</row>
</thead>
<tbody>
<row>
<entry><type>abstime</type></entry>
<entry><type>AbsoluteTime</type></entry>
<entry><filename>utils/nabstime.h</filename></entry>
</row>
<row>
<entry><type>boolean</type></entry>
<entry><type>bool</type></entry>
<entry><filename>postgres.h</filename> (maybe compiler built-in)</entry>
</row>
<row>
<entry><type>box</type></entry>
<entry><type>BOX*</type></entry>
<entry><filename>utils/geo_decls.h</filename></entry>
</row>
<row>
<entry><type>bytea</type></entry>
<entry><type>bytea*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>"char"</type></entry>
<entry><type>char</type></entry>
<entry>(compiler built-in)</entry>
</row>
<row>
<entry><type>character</type></entry>
<entry><type>BpChar*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>cid</type></entry>
<entry><type>CommandId</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>date</type></entry>
<entry><type>DateADT</type></entry>
<entry><filename>utils/date.h</filename></entry>
</row>
<row>
<entry><type>smallint</type> (<type>int2</type>)</entry>
<entry><type>int2</type> or <type>int16</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>int2vector</type></entry>
<entry><type>int2vector*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>integer</type> (<type>int4</type>)</entry>
<entry><type>int4</type> or <type>int32</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>real</type> (<type>float4</type>)</entry>
<entry><type>float4*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>double precision</type> (<type>float8</type>)</entry>
<entry><type>float8*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>interval</type></entry>
<entry><type>Interval*</type></entry>
<entry><filename>utils/timestamp.h</filename></entry>
</row>
<row>
<entry><type>lseg</type></entry>
<entry><type>LSEG*</type></entry>
<entry><filename>utils/geo_decls.h</filename></entry>
</row>
<row>
<entry><type>name</type></entry>
<entry><type>Name</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>oid</type></entry>
<entry><type>Oid</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>oidvector</type></entry>
<entry><type>oidvector*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>path</type></entry>
<entry><type>PATH*</type></entry>
<entry><filename>utils/geo_decls.h</filename></entry>
</row>
<row>
<entry><type>point</type></entry>
<entry><type>POINT*</type></entry>
<entry><filename>utils/geo_decls.h</filename></entry>
</row>
<row>
<entry><type>regproc</type></entry>
<entry><type>regproc</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>reltime</type></entry>
<entry><type>RelativeTime</type></entry>
<entry><filename>utils/nabstime.h</filename></entry>
</row>
<row>
<entry><type>text</type></entry>
<entry><type>text*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>tid</type></entry>
<entry><type>ItemPointer</type></entry>
<entry><filename>storage/itemptr.h</filename></entry>
</row>
<row>
<entry><type>time</type></entry>
<entry><type>TimeADT</type></entry>
<entry><filename>utils/date.h</filename></entry>
</row>
<row>
<entry><type>time with time zone</type></entry>
<entry><type>TimeTzADT</type></entry>
<entry><filename>utils/date.h</filename></entry>
</row>
<row>
<entry><type>timestamp</type></entry>
<entry><type>Timestamp*</type></entry>
<entry><filename>utils/timestamp.h</filename></entry>
</row>
<row>
<entry><type>tinterval</type></entry>
<entry><type>TimeInterval</type></entry>
<entry><filename>utils/nabstime.h</filename></entry>
</row>
<row>
<entry><type>varchar</type></entry>
<entry><type>VarChar*</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
<row>
<entry><type>xid</type></entry>
<entry><type>TransactionId</type></entry>
<entry><filename>postgres.h</filename></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Now that we've gone over all of the possible structures
for base types, we can show some examples of real functions.
</para>
</sect2>
<sect2>
<title>Version 0 Calling Conventions</title>
<para>
We present the <quote>old style</quote> calling convention first &mdash; although
this approach is now deprecated, it's easier to get a handle on
initially. In the version-0 method, the arguments and result
of the C function are just declared in normal C style, but being
careful to use the C representation of each SQL data type as shown
above.
</para>
<para>
Here are some examples:
<programlisting><![CDATA[
#include "postgres.h"
#include <string.h>
/* by value */
int
add_one(int arg)
{
return arg + 1;
}
/* by reference, fixed length */
float8 *
add_one_float8(float8 *arg)
{
float8 *result = (float8 *) palloc(sizeof(float8));
*result = *arg + 1.0;
return result;
}
Point *
makepoint(Point *pointx, Point *pointy)
{
Point *new_point = (Point *) palloc(sizeof(Point));
new_point->x = pointx->x;
new_point->y = pointy->y;
return new_point;
}
/* by reference, variable length */
text *
copytext(text *t)
{
/*
* VARSIZE is the total size of the struct in bytes.
*/
text *new_t = (text *) palloc(VARSIZE(t));
SET_VARSIZE(new_t, VARSIZE(t));
/*
* VARDATA is a pointer to the data region of the struct.
*/
memcpy((void *) VARDATA(new_t), /* destination */
(void *) VARDATA(t), /* source */
VARSIZE(t) - VARHDRSZ); /* how many bytes */
return new_t;
}
text *
concat_text(text *arg1, text *arg2)
{
int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
text *new_text = (text *) palloc(new_text_size);
SET_VARSIZE(new_text, new_text_size);
memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1) - VARHDRSZ);
memcpy(VARDATA(new_text) + (VARSIZE(arg1) - VARHDRSZ),
VARDATA(arg2), VARSIZE(arg2) - VARHDRSZ);
return new_text;
}
]]>
</programlisting>
</para>
<para>
Supposing that the above code has been prepared in file
<filename>funcs.c</filename> and compiled into a shared object,
we could define the functions to <productname>PostgreSQL</productname>
with commands like this:
<programlisting>
CREATE FUNCTION add_one(integer) RETURNS integer
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one'
LANGUAGE C STRICT;
-- note overloading of SQL function name "add_one"
CREATE FUNCTION add_one(double precision) RETURNS double precision
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'add_one_float8'
LANGUAGE C STRICT;
CREATE FUNCTION makepoint(point, point) RETURNS point
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'makepoint'
LANGUAGE C STRICT;
CREATE FUNCTION copytext(text) RETURNS text
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'copytext'
LANGUAGE C STRICT;
CREATE FUNCTION concat_text(text, text) RETURNS text
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'concat_text'
LANGUAGE C STRICT;
</programlisting>
</para>
<para>
Here, <replaceable>DIRECTORY</replaceable> stands for the
directory of the shared library file (for instance the
<productname>PostgreSQL</productname> tutorial directory, which
contains the code for the examples used in this section).
(Better style would be to use just <literal>'funcs'</> in the
<literal>AS</> clause, after having added
<replaceable>DIRECTORY</replaceable> to the search path. In any
case, we can omit the system-specific extension for a shared
library, commonly <literal>.so</literal> or
<literal>.sl</literal>.)
</para>
<para>
Notice that we have specified the functions as <quote>strict</quote>,
meaning that
the system should automatically assume a null result if any input
value is null. By doing this, we avoid having to check for null inputs
in the function code. Without this, we'd have to check for null values
explicitly, by checking for a null pointer for each
pass-by-reference argument. (For pass-by-value arguments, we don't
even have a way to check!)
</para>
<para>
Although this calling convention is simple to use,
it is not very portable; on some architectures there are problems
with passing data types that are smaller than <type>int</type> this way. Also, there is
no simple way to return a null result, nor to cope with null arguments
in any way other than making the function strict. The version-1
convention, presented next, overcomes these objections.
</para>
</sect2>
<sect2>
<title>Version 1 Calling Conventions</title>
<para>
The version-1 calling convention relies on macros to suppress most
of the complexity of passing arguments and results. The C declaration
of a version-1 function is always:
<programlisting>
Datum funcname(PG_FUNCTION_ARGS)
</programlisting>
In addition, the macro call:
<programlisting>
PG_FUNCTION_INFO_V1(funcname);
</programlisting>
must appear in the same source file. (Conventionally. it's
written just before the function itself.) This macro call is not
needed for <literal>internal</>-language functions, since
<productname>PostgreSQL</> assumes that all internal functions
use the version-1 convention. It is, however, required for
dynamically-loaded functions.
</para>
<para>
In a version-1 function, each actual argument is fetched using a
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
macro that corresponds to the argument's data type, and the
result is returned using a
<function>PG_RETURN_<replaceable>xxx</replaceable>()</function>
macro for the return type.
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
takes as its argument the number of the function argument to
fetch, where the count starts at 0.
<function>PG_RETURN_<replaceable>xxx</replaceable>()</function>
takes as its argument the actual value to return.
</para>
<para>
Here we show the same functions as above, coded in version-1 style:
<programlisting><![CDATA[
#include "postgres.h"
#include <string.h>
#include "fmgr.h"
/* by value */
PG_FUNCTION_INFO_V1(add_one);
Datum
add_one(PG_FUNCTION_ARGS)
{
int32 arg = PG_GETARG_INT32(0);
PG_RETURN_INT32(arg + 1);
}
/* by reference, fixed length */
PG_FUNCTION_INFO_V1(add_one_float8);
Datum
add_one_float8(PG_FUNCTION_ARGS)
{
/* The macros for FLOAT8 hide its pass-by-reference nature. */
float8 arg = PG_GETARG_FLOAT8(0);
PG_RETURN_FLOAT8(arg + 1.0);
}
PG_FUNCTION_INFO_V1(makepoint);
Datum
makepoint(PG_FUNCTION_ARGS)
{
/* Here, the pass-by-reference nature of Point is not hidden. */
Point *pointx = PG_GETARG_POINT_P(0);
Point *pointy = PG_GETARG_POINT_P(1);
Point *new_point = (Point *) palloc(sizeof(Point));
new_point->x = pointx->x;
new_point->y = pointy->y;
PG_RETURN_POINT_P(new_point);
}
/* by reference, variable length */
PG_FUNCTION_INFO_V1(copytext);
Datum
copytext(PG_FUNCTION_ARGS)
{
text *t = PG_GETARG_TEXT_P(0);
/*
* VARSIZE is the total size of the struct in bytes.
*/
text *new_t = (text *) palloc(VARSIZE(t));
SET_VARSIZE(new_t, VARSIZE(t));
/*
* VARDATA is a pointer to the data region of the struct.
*/
memcpy((void *) VARDATA(new_t), /* destination */
(void *) VARDATA(t), /* source */
VARSIZE(t) - VARHDRSZ); /* how many bytes */
PG_RETURN_TEXT_P(new_t);
}
PG_FUNCTION_INFO_V1(concat_text);
Datum
concat_text(PG_FUNCTION_ARGS)
{
text *arg1 = PG_GETARG_TEXT_P(0);
text *arg2 = PG_GETARG_TEXT_P(1);
int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
text *new_text = (text *) palloc(new_text_size);
SET_VARSIZE(new_text, new_text_size);
memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1) - VARHDRSZ);
memcpy(VARDATA(new_text) + (VARSIZE(arg1) - VARHDRSZ),
VARDATA(arg2), VARSIZE(arg2) - VARHDRSZ);
PG_RETURN_TEXT_P(new_text);
}
]]>
</programlisting>
</para>
<para>
The <command>CREATE FUNCTION</command> commands are the same as
for the version-0 equivalents.
</para>
<para>
At first glance, the version-1 coding conventions might appear to
be just pointless obscurantism. They do, however, offer a number
of improvements, because the macros can hide unnecessary detail.
An example is that in coding <function>add_one_float8</>, we no longer need to
be aware that <type>float8</type> is a pass-by-reference type. Another
example is that the <literal>GETARG</> macros for variable-length types allow
for more efficient fetching of <quote>toasted</quote> (compressed or
out-of-line) values.
</para>
<para>
One big improvement in version-1 functions is better handling of null
inputs and results. The macro <function>PG_ARGISNULL(<replaceable>n</>)</function>
allows a function to test whether each input is null. (Of course, doing
this is only necessary in functions not declared <quote>strict</>.)
As with the
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function> macros,
the input arguments are counted beginning at zero. Note that one
should refrain from executing
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function> until
one has verified that the argument isn't null.
To return a null result, execute <function>PG_RETURN_NULL()</function>;
this works in both strict and nonstrict functions.
</para>
<para>
Other options provided in the new-style interface are two
variants of the
<function>PG_GETARG_<replaceable>xxx</replaceable>()</function>
macros. The first of these,
<function>PG_GETARG_<replaceable>xxx</replaceable>_COPY()</function>,
guarantees to return a copy of the specified argument that is
safe for writing into. (The normal macros will sometimes return a
pointer to a value that is physically stored in a table, which
must not be written to. Using the
<function>PG_GETARG_<replaceable>xxx</replaceable>_COPY()</function>
macros guarantees a writable result.)
The second variant consists of the
<function>PG_GETARG_<replaceable>xxx</replaceable>_SLICE()</function>
macros which take three arguments. The first is the number of the
function argument (as above). The second and third are the offset and
length of the segment to be returned. Offsets are counted from
zero, and a negative length requests that the remainder of the
value be returned. These macros provide more efficient access to
parts of large values in the case where they have storage type
<quote>external</quote>. (The storage type of a column can be specified using
<literal>ALTER TABLE <replaceable>tablename</replaceable> ALTER
COLUMN <replaceable>colname</replaceable> SET STORAGE
<replaceable>storagetype</replaceable></literal>. <replaceable>storagetype</replaceable> is one of
<literal>plain</>, <literal>external</>, <literal>extended</literal>,
or <literal>main</>.)
</para>
<para>
Finally, the version-1 function call conventions make it possible
to return set results (<xref linkend="xfunc-c-return-set">) and
implement trigger functions (<xref linkend="triggers">) and
procedural-language call handlers (<xref
linkend="plhandler">). Version-1 code is also more
portable than version-0, because it does not break restrictions
on function call protocol in the C standard. For more details
see <filename>src/backend/utils/fmgr/README</filename> in the
source distribution.
</para>
</sect2>
<sect2>
<title>Writing Code</title>
<para>
Before we turn to the more advanced topics, we should discuss
some coding rules for <productname>PostgreSQL</productname>
C-language functions. While it might be possible to load functions
written in languages other than C into
<productname>PostgreSQL</productname>, this is usually difficult
(when it is possible at all) because other languages, such as
C++, FORTRAN, or Pascal often do not follow the same calling
convention as C. That is, other languages do not pass argument
and return values between functions in the same way. For this
reason, we will assume that your C-language functions are
actually written in C.
</para>
<para>
The basic rules for writing and building C functions are as follows:
<itemizedlist>
<listitem>
<para>
Use <literal>pg_config
--includedir-server</literal><indexterm><primary>pg_config</><secondary>with user-defined C functions</></>
to find out where the <productname>PostgreSQL</> server header
files are installed on your system (or the system that your
users will be running on).
</para>
</listitem>
<listitem>
<para>
Compiling and linking your code so that it can be dynamically
loaded into <productname>PostgreSQL</productname> always
requires special flags. See <xref linkend="dfunc"> for a
detailed explanation of how to do it for your particular
operating system.
</para>
</listitem>
<listitem>
<para>
Remember to define a <quote>magic block</> for your shared library,
as described in <xref linkend="xfunc-c-dynload">.
</para>
</listitem>
<listitem>
<para>
When allocating memory, use the
<productname>PostgreSQL</productname> functions
<function>palloc</function><indexterm><primary>palloc</></> and <function>pfree</function><indexterm><primary>pfree</></>
instead of the corresponding C library functions
<function>malloc</function> and <function>free</function>.
The memory allocated by <function>palloc</function> will be
freed automatically at the end of each transaction, preventing
memory leaks.
</para>
</listitem>
<listitem>
<para>
Always zero the bytes of your structures using
<function>memset</function>. Without this, it's difficult to
support hash indexes or hash joins, as you must pick out only
the significant bits of your data structure to compute a hash.
Even if you initialize all fields of your structure, there might be
alignment padding (holes in the structure) that contain
garbage values.
</para>
</listitem>
<listitem>
<para>
Most of the internal <productname>PostgreSQL</productname>
types are declared in <filename>postgres.h</filename>, while
the function manager interfaces
(<symbol>PG_FUNCTION_ARGS</symbol>, etc.) are in
<filename>fmgr.h</filename>, so you will need to include at
least these two files. For portability reasons it's best to
include <filename>postgres.h</filename> <emphasis>first</>,
before any other system or user header files. Including
<filename>postgres.h</filename> will also include
<filename>elog.h</filename> and <filename>palloc.h</filename>
for you.
</para>
</listitem>
<listitem>
<para>
Symbol names defined within object files must not conflict
with each other or with symbols defined in the
<productname>PostgreSQL</productname> server executable. You
will have to rename your functions or variables if you get
error messages to this effect.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
&dfunc;
<sect2 id="xfunc-c-pgxs">
<title>Extension Building Infrastructure</title>
<indexterm zone="xfunc-c-pgxs">
<primary>pgxs</primary>
</indexterm>
<para>
If you are thinking about distributing your
<productname>PostgreSQL</> extension modules, setting up a
portable build system for them can be fairly difficult. Therefore
the <productname>PostgreSQL</> installation provides a build
infrastructure for extensions, called <acronym>PGXS</acronym>, so
that simple extension modules can be built simply against an
already installed server. Note that this infrastructure is not
intended to be a universal build system framework that can be used
to build all software interfacing to <productname>PostgreSQL</>;
it simply automates common build rules for simple server extension
modules. For more complicated packages, you need to write your
own build system.
</para>
<para>
To use the infrastructure for your extension, you must write a
simple makefile. In that makefile, you need to set some variables
and finally include the global <acronym>PGXS</acronym> makefile.
Here is an example that builds an extension module named
<literal>isbn_issn</literal> consisting of a shared library, an
SQL script, and a documentation text file:
<programlisting>
MODULES = isbn_issn
DATA_built = isbn_issn.sql
DOCS = README.isbn_issn
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
</programlisting>
The last three lines should always be the same. Earlier in the
file, you assign variables or add custom
<application>make</application> rules.
</para>
<para>
The following variables can be set:
<variablelist>
<varlistentry>
<term><varname>MODULES</varname></term>
<listitem>
<para>
list of shared objects to be built from source file with same
stem (do not include suffix in this list)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>DATA</varname></term>
<listitem>
<para>
random files to install into <literal><replaceable>prefix</replaceable>/share/contrib</literal>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>DATA_built</varname></term>
<listitem>
<para>
random files to install into
<literal><replaceable>prefix</replaceable>/share/contrib</literal>,
which need to be built first
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>DOCS</varname></term>
<listitem>
<para>
random files to install under
<literal><replaceable>prefix</replaceable>/doc/contrib</literal>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>SCRIPTS</varname></term>
<listitem>
<para>
script files (not binaries) to install into
<literal><replaceable>prefix</replaceable>/bin</literal>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>SCRIPTS_built</varname></term>
<listitem>
<para>
script files (not binaries) to install into
<literal><replaceable>prefix</replaceable>/bin</literal>,
which need to be built first
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>REGRESS</varname></term>
<listitem>
<para>
list of regression test cases (without suffix), see below
</para>
</listitem>
</varlistentry>
</variablelist>
or at most one of these two:
<variablelist>
<varlistentry>
<term><varname>PROGRAM</varname></term>
<listitem>
<para>
a binary program to build (list objects files in <varname>OBJS</varname>)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>MODULE_big</varname></term>
<listitem>
<para>
a shared object to build (list object files in <varname>OBJS</varname>)
</para>
</listitem>
</varlistentry>
</variablelist>
The following can also be set:
<variablelist>
<varlistentry>
<term><varname>EXTRA_CLEAN</varname></term>
<listitem>
<para>
extra files to remove in <literal>make clean</literal>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>PG_CPPFLAGS</varname></term>
<listitem>
<para>
will be added to <varname>CPPFLAGS</varname>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>PG_LIBS</varname></term>
<listitem>
<para>
will be added to <varname>PROGRAM</varname> link line
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>SHLIB_LINK</varname></term>
<listitem>
<para>
will be added to <varname>MODULE_big</varname> link line
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>PG_CONFIG</varname></term>
<listitem>
<para>
path to <application>pg_config</> program for the
<productname>PostgreSQL</productname> installation to build against
(typically just <literal>pg_config</> to use the first one in your
<varname>PATH</>)
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Put this makefile as <literal>Makefile</literal> in the directory
which holds your extension. Then you can do
<literal>make</literal> to compile, and later <literal>make
install</literal> to install your module. By default, the extension is
compiled and installed for the
<productname>PostgreSQL</productname> installation that
corresponds to the first <command>pg_config</command> program
found in your path. You can use a different installation by
setting <varname>PG_CONFIG</varname> to point to its
<command>pg_config</command> program, either within the makefile
or on the <literal>make</literal> command line.
</para>
<caution>
<para>
Changing <varname>PG_CONFIG</varname> only works when building
against <productname>PostgreSQL</productname> 8.3 or later.
With older releases it does not work to set it to anything except
<literal>pg_config</>; you must alter your <varname>PATH</>
to select the installation to build against.
</para>
</caution>
<para>
The scripts listed in the <varname>REGRESS</> variable are used for
regression testing of your module, just like <literal>make
installcheck</literal> is used for the main
<productname>PostgreSQL</productname> server. For this to work you need
to have a subdirectory named <literal>sql/</literal> in your extension's
directory, within which you put one file for each group of tests you want
to run. The files should have extension <literal>.sql</literal>, which
should not be included in the <varname>REGRESS</varname> list in the
makefile. For each test there should be a file containing the expected
result in a subdirectory named <literal>expected/</literal>, with extension
<literal>.out</literal>. The tests are run by executing <literal>make
installcheck</literal>, and the resulting output will be compared to the
expected files. The differences will be written to the file
<literal>regression.diffs</literal> in <command>diff -c</command> format.
Note that trying to run a test which is missing the expected file will be
reported as <quote>trouble</quote>, so make sure you have all expected
files.
</para>
<tip>
<para>
The easiest way of creating the expected files is creating empty files,
then carefully inspecting the result files after a test run (to be found
in the <literal>results/</literal> directory), and copying them to
<literal>expected/</literal> if they match what you want from the test.
</para>
</tip>
</sect2>
<sect2>
<title>Composite-Type Arguments</title>
<para>
Composite types do not have a fixed layout like C structures.
Instances of a composite type can contain null fields. In
addition, composite types that are part of an inheritance
hierarchy can have different fields than other members of the
same inheritance hierarchy. Therefore,
<productname>PostgreSQL</productname> provides a function
interface for accessing fields of composite types from C.
</para>
<para>
Suppose we want to write a function to answer the query:
<programlisting>
SELECT name, c_overpaid(emp, 1500) AS overpaid
FROM emp
WHERE name = 'Bill' OR name = 'Sam';
</programlisting>
Using call conventions version 0, we can define
<function>c_overpaid</> as:
<programlisting><![CDATA[
#include "postgres.h"
#include "executor/executor.h" /* for GetAttributeByName() */
bool
c_overpaid(HeapTupleHeader t, /* the current row of emp */
int32 limit)
{
bool isnull;
int32 salary;
salary = DatumGetInt32(GetAttributeByName(t, "salary", &isnull));
if (isnull)
return false;
return salary > limit;
}
]]>
</programlisting>
In version-1 coding, the above would look like this:
<programlisting><![CDATA[
#include "postgres.h"
#include "executor/executor.h" /* for GetAttributeByName() */
PG_FUNCTION_INFO_V1(c_overpaid);
Datum
c_overpaid(PG_FUNCTION_ARGS)
{
HeapTupleHeader t = PG_GETARG_HEAPTUPLEHEADER(0);
int32 limit = PG_GETARG_INT32(1);
bool isnull;
Datum salary;
salary = GetAttributeByName(t, "salary", &isnull);
if (isnull)
PG_RETURN_BOOL(false);
/* Alternatively, we might prefer to do PG_RETURN_NULL() for null salary. */
PG_RETURN_BOOL(DatumGetInt32(salary) > limit);
}
]]>
</programlisting>
</para>
<para>
<function>GetAttributeByName</function> is the
<productname>PostgreSQL</productname> system function that
returns attributes out of the specified row. It has
three arguments: the argument of type <type>HeapTupleHeader</type> passed
into
the function, the name of the desired attribute, and a
return parameter that tells whether the attribute
is null. <function>GetAttributeByName</function> returns a <type>Datum</type>
value that you can convert to the proper data type by using the
appropriate <function>DatumGet<replaceable>XXX</replaceable>()</function>
macro. Note that the return value is meaningless if the null flag is
set; always check the null flag before trying to do anything with the
result.
</para>
<para>
There is also <function>GetAttributeByNum</function>, which selects
the target attribute by column number instead of name.
</para>
<para>
The following command declares the function
<function>c_overpaid</function> in SQL:
<programlisting>
CREATE FUNCTION c_overpaid(emp, integer) RETURNS boolean
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'c_overpaid'
LANGUAGE C STRICT;
</programlisting>
Notice we have used <literal>STRICT</> so that we did not have to
check whether the input arguments were NULL.
</para>
</sect2>
<sect2>
<title>Returning Rows (Composite Types)</title>
<para>
To return a row or composite-type value from a C-language
function, you can use a special API that provides macros and
functions to hide most of the complexity of building composite
data types. To use this API, the source file must include:
<programlisting>
#include "funcapi.h"
</programlisting>
</para>
<para>
There are two ways you can build a composite data value (henceforth
a <quote>tuple</>): you can build it from an array of Datum values,
or from an array of C strings that can be passed to the input
conversion functions of the tuple's column data types. In either
case, you first need to obtain or construct a <structname>TupleDesc</>
descriptor for the tuple structure. When working with Datums, you
pass the <structname>TupleDesc</> to <function>BlessTupleDesc</>,
and then call <function>heap_form_tuple</> for each row. When working
with C strings, you pass the <structname>TupleDesc</> to
<function>TupleDescGetAttInMetadata</>, and then call
<function>BuildTupleFromCStrings</> for each row. In the case of a
function returning a set of tuples, the setup steps can all be done
once during the first call of the function.
</para>
<para>
Several helper functions are available for setting up the needed
<structname>TupleDesc</>. The recommended way to do this in most
functions returning composite values is to call:
<programlisting>
TypeFuncClass get_call_result_type(FunctionCallInfo fcinfo,
Oid *resultTypeId,
TupleDesc *resultTupleDesc)
</programlisting>
passing the same <literal>fcinfo</> struct passed to the calling function
itself. (This of course requires that you use the version-1
calling conventions.) <varname>resultTypeId</> can be specified
as <literal>NULL</> or as the address of a local variable to receive the
function's result type OID. <varname>resultTupleDesc</> should be the
address of a local <structname>TupleDesc</> variable. Check that the
result is <literal>TYPEFUNC_COMPOSITE</>; if so,
<varname>resultTupleDesc</> has been filled with the needed
<structname>TupleDesc</>. (If it is not, you can report an error along
the lines of <quote>function returning record called in context that
cannot accept type record</quote>.)
</para>
<tip>
<para>
<function>get_call_result_type</> can resolve the actual type of a
polymorphic function result; so it is useful in functions that return
scalar polymorphic results, not only functions that return composites.
The <varname>resultTypeId</> output is primarily useful for functions
returning polymorphic scalars.
</para>
</tip>
<note>
<para>
<function>get_call_result_type</> has a sibling
<function>get_expr_result_type</>, which can be used to resolve the
expected output type for a function call represented by an expression
tree. This can be used when trying to determine the result type from
outside the function itself. There is also
<function>get_func_result_type</>, which can be used when only the
function's OID is available. However these functions are not able
to deal with functions declared to return <structname>record</>, and
<function>get_func_result_type</> cannot resolve polymorphic types,
so you should preferentially use <function>get_call_result_type</>.
</para>
</note>
<para>
Older, now-deprecated functions for obtaining
<structname>TupleDesc</>s are:
<programlisting>
TupleDesc RelationNameGetTupleDesc(const char *relname)
</programlisting>
to get a <structname>TupleDesc</> for the row type of a named relation,
and:
<programlisting>
TupleDesc TypeGetTupleDesc(Oid typeoid, List *colaliases)
</programlisting>
to get a <structname>TupleDesc</> based on a type OID. This can
be used to get a <structname>TupleDesc</> for a base or
composite type. It will not work for a function that returns
<structname>record</>, however, and it cannot resolve polymorphic
types.
</para>
<para>
Once you have a <structname>TupleDesc</>, call:
<programlisting>
TupleDesc BlessTupleDesc(TupleDesc tupdesc)
</programlisting>
if you plan to work with Datums, or:
<programlisting>
AttInMetadata *TupleDescGetAttInMetadata(TupleDesc tupdesc)
</programlisting>
if you plan to work with C strings. If you are writing a function
returning set, you can save the results of these functions in the
<structname>FuncCallContext</> structure &mdash; use the
<structfield>tuple_desc</> or <structfield>attinmeta</> field
respectively.
</para>
<para>
When working with Datums, use:
<programlisting>
HeapTuple heap_form_tuple(TupleDesc tupdesc, Datum *values, bool *isnull)
</programlisting>
to build a <structname>HeapTuple</> given user data in Datum form.
</para>
<para>
When working with C strings, use:
<programlisting>
HeapTuple BuildTupleFromCStrings(AttInMetadata *attinmeta, char **values)
</programlisting>
to build a <structname>HeapTuple</> given user data
in C string form. <literal>values</literal> is an array of C strings,
one for each attribute of the return row. Each C string should be in
the form expected by the input function of the attribute data
type. In order to return a null value for one of the attributes,
the corresponding pointer in the <parameter>values</> array
should be set to <symbol>NULL</>. This function will need to
be called again for each row you return.
</para>
<para>
Once you have built a tuple to return from your function, it
must be converted into a <type>Datum</>. Use:
<programlisting>
HeapTupleGetDatum(HeapTuple tuple)
</programlisting>
to convert a <structname>HeapTuple</> into a valid Datum. This
<type>Datum</> can be returned directly if you intend to return
just a single row, or it can be used as the current return value
in a set-returning function.
</para>
<para>
An example appears in the next section.
</para>
</sect2>
<sect2 id="xfunc-c-return-set">
<title>Returning Sets</title>
<para>
There is also a special API that provides support for returning
sets (multiple rows) from a C-language function. A set-returning
function must follow the version-1 calling conventions. Also,
source files must include <filename>funcapi.h</filename>, as
above.
</para>
<para>
A set-returning function (<acronym>SRF</>) is called
once for each item it returns. The <acronym>SRF</> must
therefore save enough state to remember what it was doing and
return the next item on each call.
The structure <structname>FuncCallContext</> is provided to help
control this process. Within a function, <literal>fcinfo-&gt;flinfo-&gt;fn_extra</>
is used to hold a pointer to <structname>FuncCallContext</>
across calls.
<programlisting>
typedef struct
{
/*
* Number of times we've been called before
*
* call_cntr is initialized to 0 for you by SRF_FIRSTCALL_INIT(), and
* incremented for you every time SRF_RETURN_NEXT() is called.
*/
uint32 call_cntr;
/*
* OPTIONAL maximum number of calls
*
* max_calls is here for convenience only and setting it is optional.
* If not set, you must provide alternative means to know when the
* function is done.
*/
uint32 max_calls;
/*
* OPTIONAL pointer to result slot
*
* This is obsolete and only present for backwards compatibility, viz,
* user-defined SRFs that use the deprecated TupleDescGetSlot().
*/
TupleTableSlot *slot;
/*
* OPTIONAL pointer to miscellaneous user-provided context information
*
* user_fctx is for use as a pointer to your own data to retain
* arbitrary context information between calls of your function.
*/
void *user_fctx;
/*
* OPTIONAL pointer to struct containing attribute type input metadata
*
* attinmeta is for use when returning tuples (i.e., composite data types)
* and is not used when returning base data types. It is only needed
* if you intend to use BuildTupleFromCStrings() to create the return
* tuple.
*/
AttInMetadata *attinmeta;
/*
* memory context used for structures that must live for multiple calls
*
* multi_call_memory_ctx is set by SRF_FIRSTCALL_INIT() for you, and used
* by SRF_RETURN_DONE() for cleanup. It is the most appropriate memory
* context for any memory that is to be reused across multiple calls
* of the SRF.
*/
MemoryContext multi_call_memory_ctx;
/*
* OPTIONAL pointer to struct containing tuple description
*
* tuple_desc is for use when returning tuples (i.e., composite data types)
* and is only needed if you are going to build the tuples with
* heap_form_tuple() rather than with BuildTupleFromCStrings(). Note that
* the TupleDesc pointer stored here should usually have been run through
* BlessTupleDesc() first.
*/
TupleDesc tuple_desc;
} FuncCallContext;
</programlisting>
</para>
<para>
An <acronym>SRF</> uses several functions and macros that
automatically manipulate the <structname>FuncCallContext</>
structure (and expect to find it via <literal>fn_extra</>). Use:
<programlisting>
SRF_IS_FIRSTCALL()
</programlisting>
to determine if your function is being called for the first or a
subsequent time. On the first call (only) use:
<programlisting>
SRF_FIRSTCALL_INIT()
</programlisting>
to initialize the <structname>FuncCallContext</>. On every function call,
including the first, use:
<programlisting>
SRF_PERCALL_SETUP()
</programlisting>
to properly set up for using the <structname>FuncCallContext</>
and clearing any previously returned data left over from the
previous pass.
</para>
<para>
If your function has data to return, use:
<programlisting>
SRF_RETURN_NEXT(funcctx, result)
</programlisting>
to return it to the caller. (<literal>result</> must be of type
<type>Datum</>, either a single value or a tuple prepared as
described above.) Finally, when your function is finished
returning data, use:
<programlisting>
SRF_RETURN_DONE(funcctx)
</programlisting>
to clean up and end the <acronym>SRF</>.
</para>
<para>
The memory context that is current when the <acronym>SRF</> is called is
a transient context that will be cleared between calls. This means
that you do not need to call <function>pfree</> on everything
you allocated using <function>palloc</>; it will go away anyway. However, if you want to allocate
any data structures to live across calls, you need to put them somewhere
else. The memory context referenced by
<structfield>multi_call_memory_ctx</> is a suitable location for any
data that needs to survive until the <acronym>SRF</> is finished running. In most
cases, this means that you should switch into
<structfield>multi_call_memory_ctx</> while doing the first-call setup.
</para>
<para>
A complete pseudo-code example looks like the following:
<programlisting>
Datum
my_set_returning_function(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
Datum result;
MemoryContext oldcontext;
<replaceable>further declarations as needed</replaceable>
if (SRF_IS_FIRSTCALL())
{
funcctx = SRF_FIRSTCALL_INIT();
oldcontext = MemoryContextSwitchTo(funcctx-&gt;multi_call_memory_ctx);
/* One-time setup code appears here: */
<replaceable>user code</replaceable>
<replaceable>if returning composite</replaceable>
<replaceable>build TupleDesc, and perhaps AttInMetadata</replaceable>
<replaceable>endif returning composite</replaceable>
<replaceable>user code</replaceable>
MemoryContextSwitchTo(oldcontext);
}
/* Each-time setup code appears here: */
<replaceable>user code</replaceable>
funcctx = SRF_PERCALL_SETUP();
<replaceable>user code</replaceable>
/* this is just one way we might test whether we are done: */
if (funcctx-&gt;call_cntr &lt; funcctx-&gt;max_calls)
{
/* Here we want to return another item: */
<replaceable>user code</replaceable>
<replaceable>obtain result Datum</replaceable>
SRF_RETURN_NEXT(funcctx, result);
}
else
{
/* Here we are done returning items and just need to clean up: */
<replaceable>user code</replaceable>
SRF_RETURN_DONE(funcctx);
}
}
</programlisting>
</para>
<para>
A complete example of a simple <acronym>SRF</> returning a composite type
looks like:
<programlisting><![CDATA[
PG_FUNCTION_INFO_V1(retcomposite);
Datum
retcomposite(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
int call_cntr;
int max_calls;
TupleDesc tupdesc;
AttInMetadata *attinmeta;
/* stuff done only on the first call of the function */
if (SRF_IS_FIRSTCALL())
{
MemoryContext oldcontext;
/* create a function context for cross-call persistence */
funcctx = SRF_FIRSTCALL_INIT();
/* switch to memory context appropriate for multiple function calls */
oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
/* total number of tuples to be returned */
funcctx->max_calls = PG_GETARG_UINT32(0);
/* Build a tuple descriptor for our result type */
if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("function returning record called in context "
"that cannot accept type record")));
/*
* generate attribute metadata needed later to produce tuples from raw
* C strings
*/
attinmeta = TupleDescGetAttInMetadata(tupdesc);
funcctx->attinmeta = attinmeta;
MemoryContextSwitchTo(oldcontext);
}
/* stuff done on every call of the function */
funcctx = SRF_PERCALL_SETUP();
call_cntr = funcctx->call_cntr;
max_calls = funcctx->max_calls;
attinmeta = funcctx->attinmeta;
if (call_cntr < max_calls) /* do when there is more left to send */
{
char **values;
HeapTuple tuple;
Datum result;
/*
* Prepare a values array for building the returned tuple.
* This should be an array of C strings which will
* be processed later by the type input functions.
*/
values = (char **) palloc(3 * sizeof(char *));
values[0] = (char *) palloc(16 * sizeof(char));
values[1] = (char *) palloc(16 * sizeof(char));
values[2] = (char *) palloc(16 * sizeof(char));
snprintf(values[0], 16, "%d", 1 * PG_GETARG_INT32(1));
snprintf(values[1], 16, "%d", 2 * PG_GETARG_INT32(1));
snprintf(values[2], 16, "%d", 3 * PG_GETARG_INT32(1));
/* build a tuple */
tuple = BuildTupleFromCStrings(attinmeta, values);
/* make the tuple into a datum */
result = HeapTupleGetDatum(tuple);
/* clean up (this is not really necessary) */
pfree(values[0]);
pfree(values[1]);
pfree(values[2]);
pfree(values);
SRF_RETURN_NEXT(funcctx, result);
}
else /* do when there is no more left */
{
SRF_RETURN_DONE(funcctx);
}
}
]]>
</programlisting>
One way to declare this function in SQL is:
<programlisting>
CREATE TYPE __retcomposite AS (f1 integer, f2 integer, f3 integer);
CREATE OR REPLACE FUNCTION retcomposite(integer, integer)
RETURNS SETOF __retcomposite
AS '<replaceable>filename</>', 'retcomposite'
LANGUAGE C IMMUTABLE STRICT;
</programlisting>
A different way is to use OUT parameters:
<programlisting>
CREATE OR REPLACE FUNCTION retcomposite(IN integer, IN integer,
OUT f1 integer, OUT f2 integer, OUT f3 integer)
RETURNS SETOF record
AS '<replaceable>filename</>', 'retcomposite'
LANGUAGE C IMMUTABLE STRICT;
</programlisting>
Notice that in this method the output type of the function is formally
an anonymous <structname>record</> type.
</para>
<para>
The directory <filename>contrib/tablefunc</> in the source
distribution contains more examples of set-returning functions.
</para>
</sect2>
<sect2>
<title>Polymorphic Arguments and Return Types</title>
<para>
C-language functions can be declared to accept and
return the polymorphic types
<type>anyelement</type>, <type>anyarray</type>, <type>anynonarray</type>,
and <type>anyenum</type>.
See <xref linkend="extend-types-polymorphic"> for a more detailed explanation
of polymorphic functions. When function arguments or return types
are defined as polymorphic types, the function author cannot know
in advance what data type it will be called with, or
need to return. There are two routines provided in <filename>fmgr.h</>
to allow a version-1 C function to discover the actual data types
of its arguments and the type it is expected to return. The routines are
called <literal>get_fn_expr_rettype(FmgrInfo *flinfo)</> and
<literal>get_fn_expr_argtype(FmgrInfo *flinfo, int argnum)</>.
They return the result or argument type OID, or <symbol>InvalidOid</symbol> if the
information is not available.
The structure <literal>flinfo</> is normally accessed as
<literal>fcinfo-&gt;flinfo</>. The parameter <literal>argnum</>
is zero based. <function>get_call_result_type</> can also be used
as an alternative to <function>get_fn_expr_rettype</>.
</para>
<para>
For example, suppose we want to write a function to accept a single
element of any type, and return a one-dimensional array of that type:
<programlisting>
PG_FUNCTION_INFO_V1(make_array);
Datum
make_array(PG_FUNCTION_ARGS)
{
ArrayType *result;
Oid element_type = get_fn_expr_argtype(fcinfo-&gt;flinfo, 0);
Datum element;
bool isnull;
int16 typlen;
bool typbyval;
char typalign;
int ndims;
int dims[MAXDIM];
int lbs[MAXDIM];
if (!OidIsValid(element_type))
elog(ERROR, "could not determine data type of input");
/* get the provided element, being careful in case it's NULL */
isnull = PG_ARGISNULL(0);
if (isnull)
element = (Datum) 0;
else
element = PG_GETARG_DATUM(0);
/* we have one dimension */
ndims = 1;
/* and one element */
dims[0] = 1;
/* and lower bound is 1 */
lbs[0] = 1;
/* get required info about the element type */
get_typlenbyvalalign(element_type, &amp;typlen, &amp;typbyval, &amp;typalign);
/* now build the array */
result = construct_md_array(&amp;element, &amp;isnull, ndims, dims, lbs,
element_type, typlen, typbyval, typalign);
PG_RETURN_ARRAYTYPE_P(result);
}
</programlisting>
</para>
<para>
The following command declares the function
<function>make_array</function> in SQL:
<programlisting>
CREATE FUNCTION make_array(anyelement) RETURNS anyarray
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'make_array'
LANGUAGE C IMMUTABLE;
</programlisting>
</para>
<para>
There is a variant of polymorphism that is only available to C-language
functions: they can be declared to take parameters of type
<literal>"any"</>. (Note that this type name must be double-quoted,
since it's also a SQL reserved word.) This works like
<type>anyelement</> except that it does not constrain different
<literal>"any"</> arguments to be the same type, nor do they help
determine the function's result type. A C-language function can also
declare its final parameter to be <literal>VARIADIC "any"</>. This will
match one or more actual arguments of any type (not necessarily the same
type). These arguments will <emphasis>not</> be gathered into an array
as happens with normal variadic functions; they will just be passed to
the function separately. The <function>PG_NARGS()</> macro and the
methods described above must be used to determine the number of actual
arguments and their types when using this feature.
</para>
</sect2>
<sect2>
<title>Shared Memory and LWLocks</title>
<para>
Add-ins can reserve LWLocks and an allocation of shared memory on server
startup. The add-in's shared library must be preloaded by specifying
it in
<xref linkend="guc-shared-preload-libraries"><indexterm><primary>shared-preload-libraries</></>.
Shared memory is reserved by calling:
<programlisting>
void RequestAddinShmemSpace(int size)
</programlisting>
from your <function>_PG_init</> function.
</para>
<para>
LWLocks are reserved by calling:
<programlisting>
void RequestAddinLWLocks(int n)
</programlisting>
from <function>_PG_init</>.
</para>
<para>
To avoid possible race-conditions, each backend should use the LWLock
<function>AddinShmemInitLock</> when connecting to and initializing
its allocation of shared memory, as shown here:
<programlisting>
static mystruct *ptr = NULL;
if (!ptr)
{
bool found;
LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
ptr = ShmemInitStruct("my struct name", size, &amp;found);
if (!ptr)
elog(ERROR, "out of shared memory");
if (!found)
{
initialize contents of shmem area;
acquire any requested LWLocks using:
ptr->mylockid = LWLockAssign();
}
LWLockRelease(AddinShmemInitLock);
}
</programlisting>
</para>
</sect2>
</sect1>
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