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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Expressions</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="PL/pgSQL - SQL Procedural Language" HREF="plpgsql.html"><LINK REL="PREVIOUS" TITLE="Declarations" HREF="plpgsql-declarations.html"><LINK REL="NEXT" TITLE="Basic Statements" HREF="plpgsql-statements.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-11-06T22:43:11"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Declarations" HREF="plpgsql-declarations.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="plpgsql.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 39. <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > - <ACRONYM CLASS="ACRONYM" >SQL</ACRONYM > Procedural Language</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Basic Statements" HREF="plpgsql-statements.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="PLPGSQL-EXPRESSIONS" >39.4. Expressions</A ></H1 ><P > All expressions used in <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > statements are processed using the server's main <ACRONYM CLASS="ACRONYM" >SQL</ACRONYM > executor. For example, when you write a <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > statement like </P><PRE CLASS="SYNOPSIS" >IF <TT CLASS="REPLACEABLE" ><I >expression</I ></TT > THEN ...</PRE ><P> <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > will evaluate the expression by feeding a query like </P><PRE CLASS="SYNOPSIS" >SELECT <TT CLASS="REPLACEABLE" ><I >expression</I ></TT ></PRE ><P> to the main SQL engine. While forming the <TT CLASS="COMMAND" >SELECT</TT > command, any occurrences of <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > variable names are replaced by parameters, as discussed in detail in <A HREF="plpgsql-implementation.html#PLPGSQL-VAR-SUBST" >Section 39.10.1</A >. This allows the query plan for the <TT CLASS="COMMAND" >SELECT</TT > to be prepared just once and then reused for subsequent evaluations with different values of the variables. Thus, what really happens on first use of an expression is essentially a <TT CLASS="COMMAND" >PREPARE</TT > command. For example, if we have declared two integer variables <TT CLASS="LITERAL" >x</TT > and <TT CLASS="LITERAL" >y</TT >, and we write </P><PRE CLASS="PROGRAMLISTING" >IF x < y THEN ...</PRE ><P> what happens behind the scenes is equivalent to </P><PRE CLASS="PROGRAMLISTING" >PREPARE <TT CLASS="REPLACEABLE" ><I >statement_name</I ></TT >(integer, integer) AS SELECT $1 < $2;</PRE ><P> and then this prepared statement is <TT CLASS="COMMAND" >EXECUTE</TT >d for each execution of the <TT CLASS="COMMAND" >IF</TT > statement, with the current values of the <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > variables supplied as parameter values. Normally these details are not important to a <SPAN CLASS="APPLICATION" >PL/pgSQL</SPAN > user, but they are useful to know when trying to diagnose a problem. More information appears in <A HREF="plpgsql-implementation.html#PLPGSQL-PLAN-CACHING" >Section 39.10.2</A >. </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="plpgsql-declarations.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="plpgsql-statements.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Declarations</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="plpgsql.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Basic Statements</TD ></TR ></TABLE ></DIV ></BODY ></HTML >