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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>43.4. 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="43.3. Declarations" /><link rel="next" href="plpgsql-statements.html" title="43.5. 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">43.4. Expressions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="plpgsql-declarations.html" title="43.3. Declarations">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="plpgsql.html" title="Chapter 43. PL/pgSQL — SQL Procedural Language">Up</a></td><th width="60%" align="center">Chapter 43. <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 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="plpgsql-statements.html" title="43.5. 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">43.4. 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 main5     <acronym class="acronym">SQL</acronym> executor.  For example, when you write6     a <span class="application">PL/pgSQL</span> statement like7</p><pre class="synopsis">8IF <em class="replaceable"><code>expression</code></em> THEN ...9</pre><p>10     <span class="application">PL/pgSQL</span> will evaluate the expression by11     feeding a query like12</p><pre class="synopsis">13SELECT <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 names17     are replaced by query parameters, as discussed in detail in18     <a class="xref" href="plpgsql-implementation.html#PLPGSQL-VAR-SUBST" title="43.11.1. Variable Substitution">Section 43.11.1</a>.19     This allows the query plan for the <code class="command">SELECT</code> to20     be prepared just once and then reused for subsequent21     evaluations with different values of the variables.  Thus, what22     really happens on first use of an expression is essentially a23     <code class="command">PREPARE</code> command.  For example, if we have declared24     two integer variables <code class="literal">x</code> and <code class="literal">y</code>, and we write25</p><pre class="programlisting">26IF x &lt; y THEN ...27</pre><p>28     what happens behind the scenes is equivalent to29</p><pre class="programlisting">30PREPARE <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 each33     execution of the <code class="command">IF</code> statement, with the current values34     of the <span class="application">PL/pgSQL</span> variables supplied as35     parameter values.  Normally these details are36     not important to a <span class="application">PL/pgSQL</span> user, but37     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="43.11.2. Plan Caching">Section 43.11.2</a>.39    </p><p>40     Since an <em class="replaceable"><code>expression</code></em> is converted to a41     <code class="literal">SELECT</code> command, it can contain the same clauses42     that an ordinary <code class="literal">SELECT</code> would, except that it43     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 with46</p><pre class="programlisting">47IF 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 as51     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 not53     more than one row.  (If it produces no rows, the result is taken as54     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="43.3. Declarations">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="plpgsql.html" title="Chapter 43. PL/pgSQL — SQL Procedural Language">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="plpgsql-statements.html" title="43.5. Basic Statements">Next</a></td></tr><tr><td width="40%" align="left" valign="top">43.3. Declarations </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 43.5. Basic Statements</td></tr></table></div></body></html>