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>Chapter 38. <SPAN
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>PL/pgSQL</SPAN
> - <ACRONYM
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> Procedural Language</TD
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>38.4. Expressions</A
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><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 38.10.1</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 &lt; y THEN ...</PRE
><P>
     what happens behind the scenes is
</P><PRE
CLASS="PROGRAMLISTING"
>PREPARE <TT
CLASS="REPLACEABLE"
><I
>statement_name</I
></TT
>(integer, integer) AS SELECT $1 &lt; $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.
     The query plan prepared in this way is saved for the life of the database
     connection, as described in
     <A
HREF="plpgsql-implementation.html#PLPGSQL-PLAN-CACHING"
>Section 38.10.2</A
>.  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.
    </P
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