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>Chapter 39. <SPAN
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> Procedural Language</TD
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>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
>.
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