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Add documentation for new plpgsql cursor operations. Also, another 

round of editorial effort.
This commit is contained in:
Tom Lane 2001-11-15 23:32:39 +00:00
parent 4be20187ab
commit fa09b6d7b2
1 changed files with 355 additions and 140 deletions
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doc/src/sgml/plsql.sgml
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@ -1,5 +1,5 @@
<!-- <!--
$Header: /cvsroot/pgsql/doc/src/sgml/Attic/plsql.sgml,v 2.47 2001/11/15 06:25:22 tgl Exp $ $Header: /cvsroot/pgsql/doc/src/sgml/Attic/plsql.sgml,v 2.48 2001/11/15 23:32:39 tgl Exp $
--> -->
<chapter id="plpgsql"> <chapter id="plpgsql">
@ -326,8 +326,14 @@ END;
<title>Lexical Details</title> <title>Lexical Details</title>
<para> <para>
All keywords and identifiers can be used in mixed upper and Each statement and declaration within a block is terminated
lower-case. by a semicolon.
</para>
<para>
All keywords and identifiers can be written in mixed upper- and
lower-case. Identifiers are implicitly converted to lower-case
unless double-quoted.
</para> </para>
<para> <para>
@ -391,7 +397,7 @@ url VARCHAR;
</para> </para>
<para> <para>
The default value is evaluated every time the function is called. So, The default value is evaluated every time the block is entered. So,
for example, assigning '<literal>now</literal>' to a variable of type for example, assigning '<literal>now</literal>' to a variable of type
<type>timestamp</type> causes the variable to have the <type>timestamp</type> causes the variable to have the
time of the current function call, not when the function was time of the current function call, not when the function was
@ -408,13 +414,21 @@ user_id CONSTANT INTEGER := 10;
</para> </para>
<sect2 id="plpgsql-declaration-aliases"> <sect2 id="plpgsql-declaration-aliases">
<title>Aliases for Parameters Passed to Functions</title> <title>Aliases for Function Parameters</title>
<para>
<synopsis>
<replaceable>name</replaceable> ALIAS FOR <replaceable>$n</replaceable>;
</synopsis>
</para>
<para> <para>
Parameters passed to functions are named with the identifiers Parameters passed to functions are named with the identifiers
<literal>$1</literal>, <literal>$2</literal>, <literal>$1</literal>, <literal>$2</literal>,
etc. Optionally, aliases can be declared for the <literal>$n</literal> etc. Optionally, aliases can be declared for <literal>$n</literal>
parameter names for increased readability. Some examples: parameter names for increased readability. Either the alias or the
numeric identifier can then be used to refer to the parameter value.
Some examples:
<programlisting> <programlisting>
CREATE FUNCTION sales_tax(REAL) RETURNS REAL AS ' CREATE FUNCTION sales_tax(REAL) RETURNS REAL AS '
DECLARE DECLARE
@ -490,6 +504,12 @@ END;
to, <emphasis>it has no</> substructure, and any attempt to access a to, <emphasis>it has no</> substructure, and any attempt to access a
field in it will draw a runtime error. field in it will draw a runtime error.
</para> </para>
<para>
Note that <literal>RECORD</> is not a true datatype, only a placeholder.
Thus, for example, one cannot declare a function returning
<literal>RECORD</>.
</para>
</sect2> </sect2>
<sect2 id="plpgsql-declaration-attributes"> <sect2 id="plpgsql-declaration-attributes">
@ -555,7 +575,7 @@ BEGIN
user_id := users_rec.user_id; user_id := users_rec.user_id;
... ...
CREATE FUNCTION cs_refresh_one_mv(INTEGER) RETURNS INTEGER AS ' CREATE FUNCTION does_view_exist(INTEGER) RETURNS bool AS '
DECLARE DECLARE
key ALIAS FOR $1; key ALIAS FOR $1;
table_data cs_materialized_views%ROWTYPE; table_data cs_materialized_views%ROWTYPE;
@ -564,18 +584,10 @@ CREATE FUNCTION cs_refresh_one_mv(INTEGER) RETURNS INTEGER AS '
WHERE sort_key=key; WHERE sort_key=key;
IF NOT FOUND THEN IF NOT FOUND THEN
RAISE EXCEPTION ''View % not found'', key; RETURN false;
RETURN 0;
END IF; END IF;
RETURN true;
-- The mv_name column of cs_materialized_views stores view END;
-- names.
EXECUTE ''TRUNCATE TABLE '' || quote_ident(table_data.mv_name);
EXECUTE ''INSERT INTO '' || quote_ident(table_data.mv_name) || '' '' || table_data.mv_query;
return 1;
end;
' LANGUAGE 'plpgsql'; ' LANGUAGE 'plpgsql';
</programlisting> </programlisting>
</listitem> </listitem>
@ -584,21 +596,21 @@ end;
</sect2> </sect2>
<sect2 id="plpgsql-declaration-renaming-vars"> <sect2 id="plpgsql-declaration-renaming-vars">
<title> <title>RENAME</title>
RENAME
</title>
<para> <para>
<synopsis>
RENAME <replaceable>oldname</replaceable> TO <replaceable>newname</replaceable>;
</synopsis>
Using the RENAME declaration you can change the name of a variable, Using the RENAME declaration you can change the name of a variable,
record or row. This is useful if NEW or OLD should be referenced record or row. This is primarily useful if NEW or OLD should be
by another name inside a trigger procedure. See also ALIAS. referenced by another name inside a trigger procedure. See also ALIAS.
</para> </para>
<para> <para>
Syntax and examples: Examples:
<programlisting> <programlisting>
RENAME <replaceable>oldname</replaceable> TO <replaceable>newname</replaceable>;
RENAME id TO user_id; RENAME id TO user_id;
RENAME this_var TO that_var; RENAME this_var TO that_var;
</programlisting> </programlisting>
@ -702,18 +714,20 @@ CREATE FUNCTION logfunc2 (TEXT) RETURNS TIMESTAMP AS '
</sect1> </sect1>
<sect1 id="plpgsql-statements"> <sect1 id="plpgsql-statements">
<title>Statements</title> <title>Basic Statements</title>
<para> <para>
This section describes all the statement types that are explicitly In this section and the following ones, we describe all the statement
understood by <application>PL/pgSQL</application>. Anything not types that are explicitly understood by
recognized as one of these statement types is presumed to be an SQL <application>PL/pgSQL</application>.
query, and is sent to the main database engine to execute (after Anything not recognized as one of these statement types is presumed
substitution for any <application>PL/pgSQL</application> variables to be an SQL query, and is sent to the main database engine to execute
(after substitution for any <application>PL/pgSQL</application> variables
used in the statement). Thus, used in the statement). Thus,
for example, SQL <command>INSERT</>, <command>UPDATE</>, and for example, SQL <command>INSERT</>, <command>UPDATE</>, and
<command>DELETE</> commands may be considered to be statements of <command>DELETE</> commands may be considered to be statements of
<application>PL/pgSQL</application>. <application>PL/pgSQL</application>. But they are not specifically
listed here.
</para> </para>
<sect2 id="plpgsql-statements-assignment"> <sect2 id="plpgsql-statements-assignment">
@ -750,8 +764,8 @@ tax := subtotal * 0.06;
</para> </para>
</sect2> </sect2>
<sect2 id="plpgsql-query-assignment"> <sect2 id="plpgsql-select-into">
<title>Query Assignments</title> <title>SELECT INTO</title>
<para> <para>
The result of a SELECT command yielding multiple columns (but The result of a SELECT command yielding multiple columns (but
@ -768,7 +782,7 @@ SELECT INTO <replaceable>target</replaceable> <replaceable>expressions</replacea
Postgres' normal interpretation of SELECT INTO, which is that the Postgres' normal interpretation of SELECT INTO, which is that the
INTO target is a newly created table. (If you want to create a INTO target is a newly created table. (If you want to create a
table from a SELECT result inside a <application>PL/pgSQL</application> function, use the table from a SELECT result inside a <application>PL/pgSQL</application> function, use the
equivalent syntax <command>CREATE TABLE AS SELECT</command>.) syntax <command>CREATE TABLE ... AS SELECT</command>.)
</para> </para>
<para> <para>
@ -779,7 +793,7 @@ SELECT INTO <replaceable>target</replaceable> <replaceable>expressions</replacea
</para> </para>
<para> <para>
Except for the INTO clause, the SELECT command is the same as a normal Except for the INTO clause, the SELECT statement is the same as a normal
SQL SELECT query and can use the full power of SELECT. SQL SELECT query and can use the full power of SELECT.
</para> </para>
@ -845,7 +859,7 @@ END;
<application>PL/pgSQL</application>, use the PERFORM statement: <application>PL/pgSQL</application>, use the PERFORM statement:
<synopsis> <synopsis>
PERFORM <replaceable>query</replaceable> PERFORM <replaceable>query</replaceable>;
</synopsis> </synopsis>
This executes a <literal>SELECT</literal> This executes a <literal>SELECT</literal>
@ -886,7 +900,7 @@ PERFORM create_mv(''cs_session_page_requests_mv'',''
is provided: is provided:
<synopsis> <synopsis>
EXECUTE <replaceable class="command">query-string</replaceable> EXECUTE <replaceable class="command">query-string</replaceable>;
</synopsis> </synopsis>
where <replaceable>query-string</replaceable> is an expression where <replaceable>query-string</replaceable> is an expression
@ -995,7 +1009,7 @@ END;
<para> <para>
<synopsis> <synopsis>
GET DIAGNOSTICS <replaceable>variable</replaceable> = <replaceable>item</replaceable> <optional> , ... </optional> GET DIAGNOSTICS <replaceable>variable</replaceable> = <replaceable>item</replaceable> <optional> , ... </optional> ;
</synopsis> </synopsis>
This command allows retrieval of system status indicators. Each This command allows retrieval of system status indicators. Each
@ -1010,12 +1024,11 @@ GET DIAGNOSTICS <replaceable>variable</replaceable> = <replaceable>item</replace
is only useful after an INSERT query. is only useful after an INSERT query.
</para> </para>
</sect2> </sect2>
</sect1>
<!-- **** PL/pgSQL Control Structures **** --> <sect1 id="plpgsql-control-structures">
<sect2 id="plpgsql-control-structures">
<title>Control Structures</title> <title>Control Structures</title>
<para> <para>
Control structures are probably the most useful (and Control structures are probably the most useful (and
important) part of <application>PL/pgSQL</>. With important) part of <application>PL/pgSQL</>. With
@ -1024,13 +1037,14 @@ GET DIAGNOSTICS <replaceable>variable</replaceable> = <replaceable>item</replace
flexible and powerful way. flexible and powerful way.
</para> </para>
<sect3 id="plpgsql-statements-returning"> <sect2 id="plpgsql-statements-returning">
<title>Returning from a function</title> <title>Returning from a function</title>
<para> <para>
<synopsis> <synopsis>
RETURN <replaceable>expression</replaceable> RETURN <replaceable>expression</replaceable>;
</synopsis> </synopsis>
The function terminates and the value of The function terminates and the value of
<replaceable>expression</replaceable> will be returned to the <replaceable>expression</replaceable> will be returned to the
upper executor. upper executor.
@ -1044,26 +1058,28 @@ RETURN <replaceable>expression</replaceable>
the function without hitting a RETURN statement, a runtime error the function without hitting a RETURN statement, a runtime error
will occur. will occur.
</para> </para>
</sect3> </sect2>
<sect3 id="plpgsql-conditionals"> <sect2 id="plpgsql-conditionals">
<title>Conditional Control: IF statements</title> <title>Conditionals</title>
<para> <para>
<function>IF</function> statements let you execute commands based on <function>IF</function> statements let you execute commands based on
certain conditions. certain conditions.
<application>PL/pgSQL</> has four forms of IF: IF-THEN, IF-THEN-ELSE, <application>PL/pgSQL</> has four forms of IF: IF-THEN, IF-THEN-ELSE,
IF-THEN-ELSE IF, IF-THEN-ELSIF-THEN-ELSE. IF-THEN-ELSE IF, and IF-THEN-ELSIF-THEN-ELSE.
</para> </para>
<variablelist> <sect3>
<varlistentry> <title>IF-THEN</title>
<term>
IF-THEN
</term>
<listitem>
<para> <para>
<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
END IF;
</synopsis>
IF-THEN statements are the simplest form of IF. The IF-THEN statements are the simplest form of IF. The
statements between THEN and END IF will be executed if statements between THEN and END IF will be executed if
the condition is true. Otherwise, they are skipped. the condition is true. Otherwise, they are skipped.
@ -1074,19 +1090,23 @@ IF v_user_id &lt;&gt; 0 THEN
END IF; END IF;
</programlisting> </programlisting>
</para> </para>
</listitem> </sect3>
</varlistentry>
<varlistentry> <sect3>
<term> <title>IF-THEN-ELSE</title>
IF-THEN-ELSE
</term>
<listitem>
<para> <para>
<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
ELSE
<replaceable>statements</replaceable>
END IF;
</synopsis>
IF-THEN-ELSE statements add to IF-THEN by letting you IF-THEN-ELSE statements add to IF-THEN by letting you
specify a group of statements that should be executed if the specify an alternative set of statements that should be executed if
condition evaluates to FALSE. the condition evaluates to FALSE.
<programlisting> <programlisting>
IF parentid IS NULL or parentid = '''' IF parentid IS NULL or parentid = ''''
@ -1105,15 +1125,11 @@ ELSE
END IF; END IF;
</programlisting> </programlisting>
</para> </para>
</listitem> </sect3>
</varlistentry>
<varlistentry> <sect3>
<term> <title>IF-THEN-ELSE IF</title>
IF-THEN-ELSE IF
</term>
<listitem>
<para> <para>
IF statements can be nested, as in the following example: IF statements can be nested, as in the following example:
<programlisting> <programlisting>
@ -1135,16 +1151,27 @@ END IF;
This is workable but grows tedious when there are many This is workable but grows tedious when there are many
alternatives to be checked. alternatives to be checked.
</para> </para>
</listitem> </sect3>
</varlistentry>
<varlistentry> <sect3>
<term> <title>IF-THEN-ELSIF-ELSE</title>
IF-THEN-ELSIF-ELSE
</term>
<listitem>
<para> <para>
<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
<replaceable>statements</replaceable>
...
</optional>
</optional>
<optional> ELSE
<replaceable>statements</replaceable> </optional>
END IF;
</synopsis>
IF-THEN-ELSIF-ELSE provides a more convenient method of checking IF-THEN-ELSIF-ELSE provides a more convenient method of checking
many alternatives in one statement. Formally it is equivalent many alternatives in one statement. Formally it is equivalent
to nested IF-THEN-ELSE-IF-THEN commands, but only one END IF to nested IF-THEN-ELSE-IF-THEN commands, but only one END IF
@ -1158,11 +1185,11 @@ END IF;
IF number = 0 THEN IF number = 0 THEN
result := ''zero''; result := ''zero'';
ELSIF number &lt; 0 THEN ELSIF number &lt; 0 THEN
result := ''negative''; result := ''positive'';
ELSIF number &gt; 0 THEN ELSIF number &gt; 0 THEN
result := ''negative''; result := ''negative'';
ELSE ELSE
-- now it seems to be NULL -- hmm, the only other possibility is that number IS NULL
result := ''NULL''; result := ''NULL'';
END IF; END IF;
</programlisting> </programlisting>
@ -1171,29 +1198,22 @@ END IF;
<para> <para>
The final ELSE section is optional. The final ELSE section is optional.
</para> </para>
</listitem>
</varlistentry>
</sect3>
</sect2>
</variablelist> <sect2 id="plpgsql-control-structures-loops">
</sect3> <title>Simple Loops</title>
<sect3 id="plpgsql-control-structures-loops">
<title>Iterative Control: LOOP, WHILE, FOR and EXIT</title>
<para> <para>
With the LOOP, WHILE, FOR and EXIT statements, you can arrange With the LOOP, EXIT, WHILE and FOR statements, you can arrange
for your <application>PL/pgSQL</application> function to repeat for your <application>PL/pgSQL</application> function to repeat
a series of commands. a series of commands.
</para> </para>
<variablelist> <sect3>
<varlistentry> <title>LOOP</title>
<term>
LOOP
</term>
<listitem>
<para> <para>
<synopsis> <synopsis>
<optional>&lt;&lt;label&gt;&gt;</optional> <optional>&lt;&lt;label&gt;&gt;</optional>
@ -1201,37 +1221,36 @@ LOOP
<replaceable>statements</replaceable> <replaceable>statements</replaceable>
END LOOP; END LOOP;
</synopsis> </synopsis>
An unconditional loop that must be terminated explicitly
by an EXIT statement. The optional label can be used by LOOP defines an unconditional loop that is repeated indefinitely
EXIT statements of nested loops to specify which level of until terminated by an EXIT or RETURN statement.
The optional label can be used by
EXIT statements in nested loops to specify which level of
nesting should be terminated. nesting should be terminated.
</para> </para>
</listitem> </sect3>
</varlistentry>
<varlistentry> <sect3>
<term> <title>EXIT</title>
EXIT
</term>
<listitem>
<para> <para>
<synopsis> <synopsis>
EXIT <optional> <replaceable>label</replaceable> </optional> <optional> WHEN <replaceable>expression</replaceable> </optional>; EXIT <optional> <replaceable>label</replaceable> </optional> <optional> WHEN <replaceable>expression</replaceable> </optional>;
</synopsis> </synopsis>
If no <replaceable>label</replaceable> is given, If no <replaceable>label</replaceable> is given,
the innermost loop is terminated and the the innermost loop is terminated and the
statement following END LOOP is executed next. statement following END LOOP is executed next.
If <replaceable>label</replaceable> is given, it If <replaceable>label</replaceable> is given, it
must be the label of the current or an outer level of nested loop must be the label of the current or some outer level of nested loop
blocks. Then the named loop or block is terminated and control or block. Then the named loop or block is terminated and control
continues with the statement after the loop's/block's corresponding continues with the statement after the loop's/block's corresponding
END. END.
</para> </para>
<para> <para>
If WHEN is present, loop exit occurs only if the specified condition If WHEN is present, loop exit occurs only if the specified condition
is true. is true, otherwise control passes to the statement after EXIT.
</para> </para>
<para> <para>
@ -1257,26 +1276,26 @@ BEGIN
END; END;
</programlisting> </programlisting>
</para> </para>
</listitem> </sect3>
</varlistentry>
<varlistentry> <sect3>
<term> <title>WHILE</title>
WHILE
</term>
<listitem>
<para> <para>
With the WHILE statement, you can repeat a
sequence of statements so long as the condition expression
evaluates to true. The condition is checked just before
each entry to the loop body.
<synopsis> <synopsis>
<optional>&lt;&lt;label&gt;&gt;</optional> <optional>&lt;&lt;label&gt;&gt;</optional>
WHILE <replaceable>expression</replaceable> LOOP WHILE <replaceable>expression</replaceable> LOOP
<replaceable>statements</replaceable> <replaceable>statements</replaceable>
END LOOP; END LOOP;
</synopsis> </synopsis>
The WHILE statement repeats a
sequence of statements so long as the condition expression
evaluates to true. The condition is checked just before
each entry to the loop body.
</para>
<para>
For example: For example:
<programlisting> <programlisting>
WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP
@ -1288,25 +1307,22 @@ WHILE NOT boolean_expression LOOP
END LOOP; END LOOP;
</programlisting> </programlisting>
</para> </para>
</listitem> </sect3>
</varlistentry>
<varlistentry> <sect3>
<term> <title>FOR (integer for-loop)</title>
FOR
</term>
<listitem>
<para> <para>
<synopsis> <synopsis>
<optional>&lt;&lt;label&gt;&gt;</optional> <optional>&lt;&lt;label&gt;&gt;</optional>
FOR <replaceable>name</replaceable> IN <optional> REVERSE </optional> <replaceable>expression</replaceable> .. <replaceable>expression</replaceable> LOOP FOR <replaceable>name</replaceable> IN <optional> REVERSE </optional> <replaceable>expression</replaceable> .. <replaceable>expression</replaceable> LOOP
<replaceable>statements</replaceable> <replaceable>statements</replaceable>
END LOOP; END LOOP;
</synopsis> </synopsis>
A loop that iterates over a range of integer values. The variable
<replaceable>name</replaceable> is automatically created as type This form of FOR creates a loop that iterates over a range of integer
values. The variable
<replaceable>name</replaceable> is automatically defined as type
integer and exists only inside the loop. The two expressions giving integer and exists only inside the loop. The two expressions giving
the lower and upper bound of the range are evaluated once when entering the lower and upper bound of the range are evaluated once when entering
the loop. The iteration step is normally 1, but is -1 when REVERSE is the loop. The iteration step is normally 1, but is -1 when REVERSE is
@ -1327,18 +1343,16 @@ FOR i IN REVERSE 10..1 LOOP
END LOOP; END LOOP;
</programlisting> </programlisting>
</para> </para>
</listitem> </sect3>
</varlistentry> </sect2>
</variablelist>
</sect3>
<sect3 id="plpgsql-records-iterating"> <sect2 id="plpgsql-records-iterating">
<title>Iterating Through Records</title> <title>Looping Through Query Results</title>
<para> <para>
Using a different type of FOR loop, you can iterate through Using a different type of FOR loop, you can iterate through
the results of a query and manipulate that data the results of a query and manipulate that data
accordingly. The syntax is as follows: accordingly. The syntax is:
<synopsis> <synopsis>
<optional>&lt;&lt;label&gt;&gt;</optional> <optional>&lt;&lt;label&gt;&gt;</optional>
FOR <replaceable>record | row</replaceable> IN <replaceable>select_query</replaceable> LOOP FOR <replaceable>record | row</replaceable> IN <replaceable>select_query</replaceable> LOOP
@ -1402,13 +1416,214 @@ END LOOP;
declared as a record/row variable. If not, it's presumed to be declared as a record/row variable. If not, it's presumed to be
an integer FOR loop. This can cause rather unintuitive error an integer FOR loop. This can cause rather unintuitive error
messages when the true problem is, say, that one has messages when the true problem is, say, that one has
misspelled the FOR variable. misspelled the FOR variable name.
</para> </para>
</note> </note>
</sect3>
</sect2> </sect2>
</sect1> </sect1>
<sect1 id="plpgsql-cursors">
<title>Cursors</title>
<para>
Rather than executing a whole query at once, it is possible to
set up a <firstterm>cursor</> that encapsulates the query, and
then read the query result a few rows at a time. One reason
for doing this is to avoid memory overrun when the result contains
a large number of rows. (However, <application>PL/pgSQL</> users
don't normally need to worry about that, since FOR loops automatically
use a cursor internally to avoid memory problems.) A more interesting
possibility is that a function can return a reference to a cursor
that it has set up, allowing the caller to read the rows. This
provides one way of returning a rowset from a function.
</para>
<sect2 id="plpgsql-cursor-declarations">
<title>Declaring Cursor Variables</title>
<para>
All access to cursors in <application>PL/pgSQL</> goes through
cursor variables, which are always of the special datatype
<type>refcursor</>. One way to create a cursor variable
is just to declare it as a variable of type <type>refcursor</>.
Another way is to use the cursor declaration syntax,
which in general is:
<synopsis>
<replaceable>name</replaceable> CURSOR <optional> ( <replaceable>arguments</replaceable> ) </optional> FOR <replaceable>select_query</replaceable> ;
</synopsis>
(<literal>FOR</> may be replaced by <literal>IS</> for Oracle
compatibility.) <replaceable>arguments</replaceable>, if any,
are a comma-separated list of <replaceable>name</replaceable>
<replaceable>datatype</replaceable> pairs that define names to
be replaced by parameter values in the given query. The actual
values to substitute for these names will be specified later,
when the cursor is opened.
</para>
<para>
Some examples:
<programlisting>
DECLARE
curs1 refcursor;
curs2 CURSOR FOR SELECT * from tenk1;
curs3 CURSOR (key int) IS SELECT * from tenk1 where unique1 = key;
</programlisting>
All three of these variables have the datatype <type>refcursor</>,
but the first may be used with any query, while the second has
a fully specified query already <firstterm>bound</> to it, and the last
has a parameterized query bound to it. (<literal>key</> will be
replaced by an integer parameter value when the cursor is opened.)
The variable <literal>curs1</>
is said to be <firstterm>unbound</> since it is not bound to
any particular query.
</para>
</sect2>
<sect2 id="plpgsql-cursor-opening">
<title>Opening Cursors</title>
<para>
Before a cursor can be used to retrieve rows, it must be
<firstterm>opened</>. (This is the equivalent action to
the SQL command <command>DECLARE CURSOR</>.)
<application>PL/pgSQL</> has four forms of the OPEN statement,
two of which are for use with unbound cursor variables
and the other two for use with bound cursor variables.
</para>
<sect3>
<title>OPEN FOR SELECT</title>
<para>
<synopsis>
OPEN <replaceable>unbound-cursor</replaceable> FOR SELECT ...;
</synopsis>
The cursor variable is opened and given the specified query
to execute. The cursor cannot be open already, and it must
have been declared as an unbound cursor (that is, as a simple
<type>refcursor</> variable). The SELECT query is treated
in the same way as other SELECTs in <application>PL/pgSQL</>:
<application>PL/pgSQL</> variable names are substituted for,
and the query plan is cached for possible re-use.
<programlisting>
OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;
</programlisting>
</para>
</sect3>
<sect3>
<title>OPEN FOR EXECUTE</title>
<para>
<synopsis>
OPEN <replaceable>unbound-cursor</replaceable> FOR EXECUTE <replaceable class="command">query-string</replaceable>;
</synopsis>
The cursor variable is opened and given the specified query
to execute. The cursor cannot be open already, and it must
have been declared as an unbound cursor (that is, as a simple
<type>refcursor</> variable). The query is specified as a
string expression in the same way as for the EXECUTE command.
As usual, this gives flexibility for the query to vary
from one run to the next.
<programlisting>
OPEN curs1 FOR EXECUTE ''SELECT * FROM '' || quote_ident($1);
</programlisting>
</para>
</sect3>
<sect3>
<title>OPENing a bound cursor</title>
<para>
<synopsis>
OPEN <replaceable>bound-cursor</replaceable> <optional> ( <replaceable>argument_values</replaceable> ) </optional>;
</synopsis>
This form of OPEN is used to open a cursor variable whose query
was bound to it when it was declared.
The cursor cannot be open already. A list of actual argument
value expressions must appear if and only if the cursor was
declared to take arguments. These values will be substituted
into the query.
The query plan for a bound cursor is always considered
cacheable --- there is no equivalent of EXECUTE in this case.
<programlisting>
OPEN curs2;
OPEN curs3(42);
</programlisting>
</para>
</sect3>
</sect2>
<sect2 id="plpgsql-cursor-using">
<title>Using Cursors</title>
<para>
Once a cursor has been opened, it can be manipulated with the
statements described here.
</para>
<para>
These manipulations need not occur in the same function that
opened the cursor to begin with. You can return a <type>refcursor</>
value out of a function and let the caller operate on the cursor.
(Internally, a <type>refcursor</> value is simply the string name
of a Portal containing the active query for the cursor. This name
can be passed around, assigned to other <type>refcursor</> variables,
and so on, without disturbing the Portal.)
</para>
<para>
All Portals are implicitly closed at end of transaction. Therefore
a <type>refcursor</> value is useful to reference an open cursor
only until the end of the transaction.
</para>
<sect3>
<title>FETCH</title>
<para>
<synopsis>
FETCH <replaceable>cursor</replaceable> INTO <replaceable>target</replaceable>;
</synopsis>
FETCH retrieves the next row from the cursor into a target,
which may be a row variable, a record variable, or a comma-separated
list of simple variables, just as for SELECT INTO. As with
SELECT INTO, the special variable FOUND may be checked to see
whether a row was obtained or not.
<programlisting>
FETCH curs1 INTO rowvar;
FETCH curs2 INTO foo,bar,baz;
</programlisting>
</para>
</sect3>
<sect3>
<title>CLOSE</title>
<para>
<synopsis>
CLOSE <replaceable>cursor</replaceable>;
</synopsis>
CLOSE closes the Portal underlying an open cursor.
This can be used to release resources earlier than end of
transaction, or to free up the cursor variable to be opened again.
<programlisting>
CLOSE curs1;
</programlisting>
</para>
</sect3>
</sect2>
</sect1>
<sect1 id="plpgsql-errors-and-messages"> <sect1 id="plpgsql-errors-and-messages">
<title>Errors and Messages</title> <title>Errors and Messages</title>