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diff --git a/doc/src/sgml/html/plpgsql-cursors.html b/doc/src/sgml/html/plpgsql-cursors.html new file mode 100644 index 0000000..f82c5c3 --- /dev/null +++ b/doc/src/sgml/html/plpgsql-cursors.html @@ -0,0 +1,386 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>42.7. Cursors</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="plpgsql-control-structures.html" title="42.6. Control Structures" /><link rel="next" href="plpgsql-transactions.html" title="42.8. Transaction Management" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">42.7. Cursors</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="plpgsql-control-structures.html" title="42.6. Control Structures">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="plpgsql.html" title="Chapter 42. PL/pgSQL — SQL Procedural Language">Up</a></td><th width="60%" align="center">Chapter 42. <span xmlns="http://www.w3.org/1999/xhtml" class="application">PL/pgSQL</span> — <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym> Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="plpgsql-transactions.html" title="42.8. Transaction Management">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="PLPGSQL-CURSORS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">42.7. Cursors</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="plpgsql-cursors.html#PLPGSQL-CURSOR-DECLARATIONS">42.7.1. Declaring Cursor Variables</a></span></dt><dt><span class="sect2"><a href="plpgsql-cursors.html#PLPGSQL-CURSOR-OPENING">42.7.2. Opening Cursors</a></span></dt><dt><span class="sect2"><a href="plpgsql-cursors.html#PLPGSQL-CURSOR-USING">42.7.3. Using Cursors</a></span></dt><dt><span class="sect2"><a href="plpgsql-cursors.html#PLPGSQL-CURSOR-FOR-LOOP">42.7.4. Looping through a Cursor's Result</a></span></dt></dl></div><a id="id-1.8.8.9.2" class="indexterm"></a><p> + Rather than executing a whole query at once, it is possible to set + up a <em class="firstterm">cursor</em> 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, <span class="application">PL/pgSQL</span> users do not normally need + to worry about that, since <code class="literal">FOR</code> loops automatically use a cursor + internally to avoid memory problems.) A more interesting usage is to + return a reference to a cursor that a function has created, allowing the + caller to read the rows. This provides an efficient way to return + large row sets from functions. + </p><div class="sect2" id="PLPGSQL-CURSOR-DECLARATIONS"><div class="titlepage"><div><div><h3 class="title">42.7.1. Declaring Cursor Variables</h3></div></div></div><p> + All access to cursors in <span class="application">PL/pgSQL</span> goes through + cursor variables, which are always of the special data type + <code class="type">refcursor</code>. One way to create a cursor variable + is just to declare it as a variable of type <code class="type">refcursor</code>. + Another way is to use the cursor declaration syntax, + which in general is: +</p><pre class="synopsis"> +<em class="replaceable"><code>name</code></em> [<span class="optional"> [<span class="optional"> NO </span>] SCROLL </span>] CURSOR [<span class="optional"> ( <em class="replaceable"><code>arguments</code></em> ) </span>] FOR <em class="replaceable"><code>query</code></em>; +</pre><p> + (<code class="literal">FOR</code> can be replaced by <code class="literal">IS</code> for + <span class="productname">Oracle</span> compatibility.) + If <code class="literal">SCROLL</code> is specified, the cursor will be capable of + scrolling backward; if <code class="literal">NO SCROLL</code> is specified, backward + fetches will be rejected; if neither specification appears, it is + query-dependent whether backward fetches will be allowed. + <em class="replaceable"><code>arguments</code></em>, if specified, is a + comma-separated list of pairs <code class="literal"><em class="replaceable"><code>name</code></em> + <em class="replaceable"><code>datatype</code></em></code> 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. + </p><p> + Some examples: +</p><pre class="programlisting"> +DECLARE + curs1 refcursor; + curs2 CURSOR FOR SELECT * FROM tenk1; + curs3 CURSOR (key integer) FOR SELECT * FROM tenk1 WHERE unique1 = key; +</pre><p> + All three of these variables have the data type <code class="type">refcursor</code>, + but the first can be used with any query, while the second has + a fully specified query already <em class="firstterm">bound</em> to it, and the last + has a parameterized query bound to it. (<code class="literal">key</code> will be + replaced by an integer parameter value when the cursor is opened.) + The variable <code class="literal">curs1</code> + is said to be <em class="firstterm">unbound</em> since it is not bound to + any particular query. + </p><p> + The <code class="literal">SCROLL</code> option cannot be used when the cursor's + query uses <code class="literal">FOR UPDATE/SHARE</code>. Also, it is + best to use <code class="literal">NO SCROLL</code> with a query that involves + volatile functions. The implementation of <code class="literal">SCROLL</code> + assumes that re-reading the query's output will give consistent + results, which a volatile function might not do. + </p></div><div class="sect2" id="PLPGSQL-CURSOR-OPENING"><div class="titlepage"><div><div><h3 class="title">42.7.2. Opening Cursors</h3></div></div></div><p> + Before a cursor can be used to retrieve rows, it must be + <em class="firstterm">opened</em>. (This is the equivalent action to the SQL + command <code class="command">DECLARE CURSOR</code>.) <span class="application">PL/pgSQL</span> has + three forms of the <code class="command">OPEN</code> statement, two of which use unbound + cursor variables while the third uses a bound cursor variable. + </p><div class="note"><h3 class="title">Note</h3><p> + Bound cursor variables can also be used without explicitly opening the cursor, + via the <code class="command">FOR</code> statement described in + <a class="xref" href="plpgsql-cursors.html#PLPGSQL-CURSOR-FOR-LOOP" title="42.7.4. Looping through a Cursor's Result">Section 42.7.4</a>. + </p></div><div class="sect3" id="id-1.8.8.9.5.4"><div class="titlepage"><div><div><h4 class="title">42.7.2.1. <code class="command">OPEN FOR</code> <em class="replaceable"><code>query</code></em></h4></div></div></div><pre class="synopsis"> +OPEN <em class="replaceable"><code>unbound_cursorvar</code></em> [<span class="optional"> [<span class="optional"> NO </span>] SCROLL </span>] FOR <em class="replaceable"><code>query</code></em>; +</pre><p> + 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 variable (that is, as a simple + <code class="type">refcursor</code> variable). The query must be a + <code class="command">SELECT</code>, or something else that returns rows + (such as <code class="command">EXPLAIN</code>). The query + is treated in the same way as other SQL commands in + <span class="application">PL/pgSQL</span>: <span class="application">PL/pgSQL</span> + variable names are substituted, and the query plan is cached for + possible reuse. When a <span class="application">PL/pgSQL</span> + variable is substituted into the cursor query, the value that is + substituted is the one it has at the time of the <code class="command">OPEN</code>; + subsequent changes to the variable will not affect the cursor's + behavior. + The <code class="literal">SCROLL</code> and <code class="literal">NO SCROLL</code> + options have the same meanings as for a bound cursor. + </p><p> + An example: +</p><pre class="programlisting"> +OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey; +</pre><p> + </p></div><div class="sect3" id="id-1.8.8.9.5.5"><div class="titlepage"><div><div><h4 class="title">42.7.2.2. <code class="command">OPEN FOR EXECUTE</code></h4></div></div></div><pre class="synopsis"> +OPEN <em class="replaceable"><code>unbound_cursorvar</code></em> [<span class="optional"> [<span class="optional"> NO </span>] SCROLL </span>] FOR EXECUTE <em class="replaceable"><code>query_string</code></em> + [<span class="optional"> USING <em class="replaceable"><code>expression</code></em> [<span class="optional">, ... </span>] </span>]; +</pre><p> + 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 variable (that is, as a simple + <code class="type">refcursor</code> variable). The query is specified as a string + expression, in the same way as in the <code class="command">EXECUTE</code> + command. As usual, this gives flexibility so the query plan can vary + from one run to the next (see <a class="xref" href="plpgsql-implementation.html#PLPGSQL-PLAN-CACHING" title="42.11.2. Plan Caching">Section 42.11.2</a>), + and it also means that variable substitution is not done on the + command string. As with <code class="command">EXECUTE</code>, parameter values + can be inserted into the dynamic command via + <code class="literal">format()</code> and <code class="literal">USING</code>. + The <code class="literal">SCROLL</code> and + <code class="literal">NO SCROLL</code> options have the same meanings as for a bound + cursor. + </p><p> + An example: +</p><pre class="programlisting"> +OPEN curs1 FOR EXECUTE format('SELECT * FROM %I WHERE col1 = $1',tabname) USING keyvalue; +</pre><p> + In this example, the table name is inserted into the query via + <code class="function">format()</code>. The comparison value for <code class="literal">col1</code> + is inserted via a <code class="literal">USING</code> parameter, so it needs + no quoting. + </p></div><div class="sect3" id="PLPGSQL-OPEN-BOUND-CURSOR"><div class="titlepage"><div><div><h4 class="title">42.7.2.3. Opening a Bound Cursor</h4></div></div></div><pre class="synopsis"> +OPEN <em class="replaceable"><code>bound_cursorvar</code></em> [<span class="optional"> ( [<span class="optional"> <em class="replaceable"><code>argument_name</code></em> := </span>] <em class="replaceable"><code>argument_value</code></em> [<span class="optional">, ...</span>] ) </span>]; +</pre><p> + This form of <code class="command">OPEN</code> is used to open a cursor + variable whose query was bound to it when it was declared. The + cursor cannot be open already. A list of actual argument value + expressions must appear if and only if the cursor was declared to + take arguments. These values will be substituted in the query. + </p><p> + The query plan for a bound cursor is always considered cacheable; + there is no equivalent of <code class="command">EXECUTE</code> in this case. + Notice that <code class="literal">SCROLL</code> and <code class="literal">NO SCROLL</code> cannot be + specified in <code class="command">OPEN</code>, as the cursor's scrolling + behavior was already determined. + </p><p> + Argument values can be passed using either <em class="firstterm">positional</em> + or <em class="firstterm">named</em> notation. In positional + notation, all arguments are specified in order. In named notation, + each argument's name is specified using <code class="literal">:=</code> to + separate it from the argument expression. Similar to calling + functions, described in <a class="xref" href="sql-syntax-calling-funcs.html" title="4.3. Calling Functions">Section 4.3</a>, it + is also allowed to mix positional and named notation. + </p><p> + Examples (these use the cursor declaration examples above): +</p><pre class="programlisting"> +OPEN curs2; +OPEN curs3(42); +OPEN curs3(key := 42); +</pre><p> + </p><p> + Because variable substitution is done on a bound cursor's query, + there are really two ways to pass values into the cursor: either + with an explicit argument to <code class="command">OPEN</code>, or implicitly by + referencing a <span class="application">PL/pgSQL</span> variable in the query. + However, only variables declared before the bound cursor was + declared will be substituted into it. In either case the value to + be passed is determined at the time of the <code class="command">OPEN</code>. + For example, another way to get the same effect as the + <code class="literal">curs3</code> example above is +</p><pre class="programlisting"> +DECLARE + key integer; + curs4 CURSOR FOR SELECT * FROM tenk1 WHERE unique1 = key; +BEGIN + key := 42; + OPEN curs4; +</pre><p> + </p></div></div><div class="sect2" id="PLPGSQL-CURSOR-USING"><div class="titlepage"><div><div><h3 class="title">42.7.3. Using Cursors</h3></div></div></div><p> + Once a cursor has been opened, it can be manipulated with the + statements described here. + </p><p> + These manipulations need not occur in the same function that + opened the cursor to begin with. You can return a <code class="type">refcursor</code> + value out of a function and let the caller operate on the cursor. + (Internally, a <code class="type">refcursor</code> value is simply the string name + of a so-called portal containing the active query for the cursor. This name + can be passed around, assigned to other <code class="type">refcursor</code> variables, + and so on, without disturbing the portal.) + </p><p> + All portals are implicitly closed at transaction end. Therefore + a <code class="type">refcursor</code> value is usable to reference an open cursor + only until the end of the transaction. + </p><div class="sect3" id="id-1.8.8.9.6.5"><div class="titlepage"><div><div><h4 class="title">42.7.3.1. <code class="literal">FETCH</code></h4></div></div></div><pre class="synopsis"> +FETCH [<span class="optional"> <em class="replaceable"><code>direction</code></em> { FROM | IN } </span>] <em class="replaceable"><code>cursor</code></em> INTO <em class="replaceable"><code>target</code></em>; +</pre><p> + <code class="command">FETCH</code> retrieves the next row from the + cursor into a target, which might be a row variable, a record + variable, or a comma-separated list of simple variables, just like + <code class="command">SELECT INTO</code>. If there is no next row, the + target is set to NULL(s). As with <code class="command">SELECT + INTO</code>, the special variable <code class="literal">FOUND</code> can + be checked to see whether a row was obtained or not. + </p><p> + The <em class="replaceable"><code>direction</code></em> clause can be any of the + variants allowed in the SQL <a class="xref" href="sql-fetch.html" title="FETCH"><span class="refentrytitle">FETCH</span></a> + command except the ones that can fetch + more than one row; namely, it can be + <code class="literal">NEXT</code>, + <code class="literal">PRIOR</code>, + <code class="literal">FIRST</code>, + <code class="literal">LAST</code>, + <code class="literal">ABSOLUTE</code> <em class="replaceable"><code>count</code></em>, + <code class="literal">RELATIVE</code> <em class="replaceable"><code>count</code></em>, + <code class="literal">FORWARD</code>, or + <code class="literal">BACKWARD</code>. + Omitting <em class="replaceable"><code>direction</code></em> is the same + as specifying <code class="literal">NEXT</code>. + In the forms using a <em class="replaceable"><code>count</code></em>, + the <em class="replaceable"><code>count</code></em> can be any integer-valued + expression (unlike the SQL <code class="command">FETCH</code> command, + which only allows an integer constant). + <em class="replaceable"><code>direction</code></em> values that require moving + backward are likely to fail unless the cursor was declared or opened + with the <code class="literal">SCROLL</code> option. + </p><p> + <em class="replaceable"><code>cursor</code></em> must be the name of a <code class="type">refcursor</code> + variable that references an open cursor portal. + </p><p> + Examples: +</p><pre class="programlisting"> +FETCH curs1 INTO rowvar; +FETCH curs2 INTO foo, bar, baz; +FETCH LAST FROM curs3 INTO x, y; +FETCH RELATIVE -2 FROM curs4 INTO x; +</pre><p> + </p></div><div class="sect3" id="id-1.8.8.9.6.6"><div class="titlepage"><div><div><h4 class="title">42.7.3.2. <code class="literal">MOVE</code></h4></div></div></div><pre class="synopsis"> +MOVE [<span class="optional"> <em class="replaceable"><code>direction</code></em> { FROM | IN } </span>] <em class="replaceable"><code>cursor</code></em>; +</pre><p> + <code class="command">MOVE</code> repositions a cursor without retrieving + any data. <code class="command">MOVE</code> works exactly like the + <code class="command">FETCH</code> command, except it only repositions the + cursor and does not return the row moved to. As with <code class="command">SELECT + INTO</code>, the special variable <code class="literal">FOUND</code> can + be checked to see whether there was a next row to move to. + </p><p> + Examples: +</p><pre class="programlisting"> +MOVE curs1; +MOVE LAST FROM curs3; +MOVE RELATIVE -2 FROM curs4; +MOVE FORWARD 2 FROM curs4; +</pre><p> + </p></div><div class="sect3" id="id-1.8.8.9.6.7"><div class="titlepage"><div><div><h4 class="title">42.7.3.3. <code class="literal">UPDATE/DELETE WHERE CURRENT OF</code></h4></div></div></div><pre class="synopsis"> +UPDATE <em class="replaceable"><code>table</code></em> SET ... WHERE CURRENT OF <em class="replaceable"><code>cursor</code></em>; +DELETE FROM <em class="replaceable"><code>table</code></em> WHERE CURRENT OF <em class="replaceable"><code>cursor</code></em>; +</pre><p> + When a cursor is positioned on a table row, that row can be updated + or deleted using the cursor to identify the row. There are + restrictions on what the cursor's query can be (in particular, + no grouping) and it's best to use <code class="literal">FOR UPDATE</code> in the + cursor. For more information see the + <a class="xref" href="sql-declare.html" title="DECLARE"><span class="refentrytitle">DECLARE</span></a> + reference page. + </p><p> + An example: +</p><pre class="programlisting"> +UPDATE foo SET dataval = myval WHERE CURRENT OF curs1; +</pre><p> + </p></div><div class="sect3" id="id-1.8.8.9.6.8"><div class="titlepage"><div><div><h4 class="title">42.7.3.4. <code class="literal">CLOSE</code></h4></div></div></div><pre class="synopsis"> +CLOSE <em class="replaceable"><code>cursor</code></em>; +</pre><p> + <code class="command">CLOSE</code> 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. + </p><p> + An example: +</p><pre class="programlisting"> +CLOSE curs1; +</pre><p> + </p></div><div class="sect3" id="id-1.8.8.9.6.9"><div class="titlepage"><div><div><h4 class="title">42.7.3.5. Returning Cursors</h4></div></div></div><p> + <span class="application">PL/pgSQL</span> functions can return cursors to the + caller. This is useful to return multiple rows or columns, + especially with very large result sets. To do this, the function + opens the cursor and returns the cursor name to the caller (or simply + opens the cursor using a portal name specified by or otherwise known + to the caller). The caller can then fetch rows from the cursor. The + cursor can be closed by the caller, or it will be closed automatically + when the transaction closes. + </p><p> + The portal name used for a cursor can be specified by the + programmer or automatically generated. To specify a portal name, + simply assign a string to the <code class="type">refcursor</code> variable before + opening it. The string value of the <code class="type">refcursor</code> variable + will be used by <code class="command">OPEN</code> as the name of the underlying portal. + However, if the <code class="type">refcursor</code> variable is null, + <code class="command">OPEN</code> automatically generates a name that does not + conflict with any existing portal, and assigns it to the + <code class="type">refcursor</code> variable. + </p><div class="note"><h3 class="title">Note</h3><p> + A bound cursor variable is initialized to the string value + representing its name, so that the portal name is the same as + the cursor variable name, unless the programmer overrides it + by assignment before opening the cursor. But an unbound cursor + variable defaults to the null value initially, so it will receive + an automatically-generated unique name, unless overridden. + </p></div><p> + The following example shows one way a cursor name can be supplied by + the caller: + +</p><pre class="programlisting"> +CREATE TABLE test (col text); +INSERT INTO test VALUES ('123'); + +CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS ' +BEGIN + OPEN $1 FOR SELECT col FROM test; + RETURN $1; +END; +' LANGUAGE plpgsql; + +BEGIN; +SELECT reffunc('funccursor'); +FETCH ALL IN funccursor; +COMMIT; +</pre><p> + </p><p> + The following example uses automatic cursor name generation: + +</p><pre class="programlisting"> +CREATE FUNCTION reffunc2() RETURNS refcursor AS ' +DECLARE + ref refcursor; +BEGIN + OPEN ref FOR SELECT col FROM test; + RETURN ref; +END; +' LANGUAGE plpgsql; + +-- need to be in a transaction to use cursors. +BEGIN; +SELECT reffunc2(); + + reffunc2 +-------------------- + <unnamed cursor 1> +(1 row) + +FETCH ALL IN "<unnamed cursor 1>"; +COMMIT; +</pre><p> + </p><p> + The following example shows one way to return multiple cursors + from a single function: + +</p><pre class="programlisting"> +CREATE FUNCTION myfunc(refcursor, refcursor) RETURNS SETOF refcursor AS $$ +BEGIN + OPEN $1 FOR SELECT * FROM table_1; + RETURN NEXT $1; + OPEN $2 FOR SELECT * FROM table_2; + RETURN NEXT $2; +END; +$$ LANGUAGE plpgsql; + +-- need to be in a transaction to use cursors. +BEGIN; + +SELECT * FROM myfunc('a', 'b'); + +FETCH ALL FROM a; +FETCH ALL FROM b; +COMMIT; +</pre><p> + </p></div></div><div class="sect2" id="PLPGSQL-CURSOR-FOR-LOOP"><div class="titlepage"><div><div><h3 class="title">42.7.4. Looping through a Cursor's Result</h3></div></div></div><p> + There is a variant of the <code class="command">FOR</code> statement that allows + iterating through the rows returned by a cursor. The syntax is: + +</p><pre class="synopsis"> +[<span class="optional"> <<<em class="replaceable"><code>label</code></em>>> </span>] +FOR <em class="replaceable"><code>recordvar</code></em> IN <em class="replaceable"><code>bound_cursorvar</code></em> [<span class="optional"> ( [<span class="optional"> <em class="replaceable"><code>argument_name</code></em> := </span>] <em class="replaceable"><code>argument_value</code></em> [<span class="optional">, ...</span>] ) </span>] LOOP + <em class="replaceable"><code>statements</code></em> +END LOOP [<span class="optional"> <em class="replaceable"><code>label</code></em> </span>]; +</pre><p> + + The cursor variable must have been bound to some query when it was + declared, and it <span class="emphasis"><em>cannot</em></span> be open already. The + <code class="command">FOR</code> statement automatically opens the cursor, and it closes + the cursor again when the loop exits. A list of actual argument value + expressions must appear if and only if the cursor was declared to take + arguments. These values will be substituted in the query, in just + the same way as during an <code class="command">OPEN</code> (see <a class="xref" href="plpgsql-cursors.html#PLPGSQL-OPEN-BOUND-CURSOR" title="42.7.2.3. Opening a Bound Cursor">Section 42.7.2.3</a>). + </p><p> + The variable <em class="replaceable"><code>recordvar</code></em> is automatically + defined as type <code class="type">record</code> and exists only inside the loop (any + existing definition of the variable name is ignored within the loop). + Each row returned by the cursor is successively assigned to this + record variable and the loop body is executed. + </p></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="plpgsql-control-structures.html" title="42.6. Control Structures">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="plpgsql.html" title="Chapter 42. PL/pgSQL — SQL Procedural Language">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="plpgsql-transactions.html" title="42.8. 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