begriffs open source - ai-pg/blob - full-docs/src/sgml/html/plpgsql-expressions.html

   1 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
   2 <!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>41.4. Expressions</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 Vsnapshot" /><link rel="prev" href="plpgsql-declarations.html" title="41.3. Declarations" /><link rel="next" href="plpgsql-statements.html" title="41.5. Basic Statements" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">41.4. Expressions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="plpgsql-declarations.html" title="41.3. Declarations">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="plpgsql.html" title="Chapter 41. PL/pgSQL — SQL Procedural Language">Up</a></td><th width="60%" align="center">Chapter 41. <span class="application">PL/pgSQL</span> — <acronym class="acronym">SQL</acronym> Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 18.0 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="plpgsql-statements.html" title="41.5. Basic Statements">Next</a></td></tr></table><hr /></div><div class="sect1" id="PLPGSQL-EXPRESSIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">41.4. Expressions <a href="#PLPGSQL-EXPRESSIONS" class="id_link">#</a></h2></div></div></div><p>
   3      All expressions used in <span class="application">PL/pgSQL</span>
   4      statements are processed using the server's main
   5      <acronym class="acronym">SQL</acronym> executor.  For example, when you write
   6      a <span class="application">PL/pgSQL</span> statement like
   7 </p><pre class="synopsis">
   8 IF <em class="replaceable"><code>expression</code></em> THEN ...
   9 </pre><p>
  10      <span class="application">PL/pgSQL</span> will evaluate the expression by
  11      feeding a query like
  12 </p><pre class="synopsis">
  13 SELECT <em class="replaceable"><code>expression</code></em>
  14 </pre><p>
  15      to the main SQL engine.  While forming the <code class="command">SELECT</code> command,
  16      any occurrences of <span class="application">PL/pgSQL</span> variable names
  17      are replaced by query parameters, as discussed in detail in
  18      <a class="xref" href="plpgsql-implementation.html#PLPGSQL-VAR-SUBST" title="41.11.1. Variable Substitution">Section 41.11.1</a>.
  19      This allows the query plan for the <code class="command">SELECT</code> to
  20      be prepared just once and then reused for subsequent
  21      evaluations with different values of the variables.  Thus, what
  22      really happens on first use of an expression is essentially a
  23      <code class="command">PREPARE</code> command.  For example, if we have declared
  24      two integer variables <code class="literal">x</code> and <code class="literal">y</code>, and we write
  25 </p><pre class="programlisting">
  26 IF x &lt; y THEN ...
  27 </pre><p>
  28      what happens behind the scenes is equivalent to
  29 </p><pre class="programlisting">
  30 PREPARE <em class="replaceable"><code>statement_name</code></em>(integer, integer) AS SELECT $1 &lt; $2;
  31 </pre><p>
  32      and then this prepared statement is <code class="command">EXECUTE</code>d for each
  33      execution of the <code class="command">IF</code> statement, with the current values
  34      of the <span class="application">PL/pgSQL</span> variables supplied as
  35      parameter values.  Normally these details are
  36      not important to a <span class="application">PL/pgSQL</span> user, but
  37      they are useful to know when trying to diagnose a problem.
  38      More information appears in <a class="xref" href="plpgsql-implementation.html#PLPGSQL-PLAN-CACHING" title="41.11.2. Plan Caching">Section 41.11.2</a>.
  39     </p><p>
  40      Since an <em class="replaceable"><code>expression</code></em> is converted to a
  41      <code class="literal">SELECT</code> command, it can contain the same clauses
  42      that an ordinary <code class="literal">SELECT</code> would, except that it
  43      cannot include a top-level <code class="literal">UNION</code>,
  44      <code class="literal">INTERSECT</code>, or <code class="literal">EXCEPT</code> clause.
  45      Thus for example one could test whether a table is non-empty with
  46 </p><pre class="programlisting">
  47 IF count(*) &gt; 0 FROM my_table THEN ...
  48 </pre><p>
  49      since the <em class="replaceable"><code>expression</code></em>
  50      between <code class="literal">IF</code> and <code class="literal">THEN</code> is parsed as
  51      though it were <code class="literal">SELECT count(*) &gt; 0 FROM my_table</code>.
  52      The <code class="literal">SELECT</code> must produce a single column, and not
  53      more than one row.  (If it produces no rows, the result is taken as
  54      NULL.)
  55     </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="plpgsql-declarations.html" title="41.3. Declarations">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="plpgsql.html" title="Chapter 41. PL/pgSQL — SQL Procedural Language">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="plpgsql-statements.html" title="41.5. Basic Statements">Next</a></td></tr><tr><td width="40%" align="left" valign="top">41.3. Declarations </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 18.0 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 41.5. Basic Statements</td></tr></table></div></body></html>