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- 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 <em class="replaceable"><code>expression</code></em> THEN ...
- </pre><p>
- <span class="application">PL/pgSQL</span> will evaluate the expression by
- feeding a query like
- </p><pre class="synopsis">
- SELECT <em class="replaceable"><code>expression</code></em>
- </pre><p>
- to the main SQL engine. While forming the <code class="command">SELECT</code> command,
- any occurrences of <span class="application">PL/pgSQL</span> variable names
- are replaced by parameters, as discussed in detail in
- <a class="xref" href="plpgsql-implementation.html#PLPGSQL-VAR-SUBST" title="42.11.1. Variable Substitution">Section 42.11.1</a>.
- This allows the query plan for the <code class="command">SELECT</code> 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
- <code class="command">PREPARE</code> command. For example, if we have declared
- two integer variables <code class="literal">x</code> and <code class="literal">y</code>, 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 <em class="replaceable"><code>statement_name</code></em>(integer, integer) AS SELECT $1 < $2;
- </pre><p>
- and then this prepared statement is <code class="command">EXECUTE</code>d for each
- execution of the <code class="command">IF</code> 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 class="xref" href="plpgsql-implementation.html#PLPGSQL-PLAN-CACHING" title="42.11.2. Plan Caching">Section 42.11.2</a>.
- </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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="plpgsql.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="plpgsql-statements.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">42.3. Declarations </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 42.5. Basic Statements</td></tr></table></div></body></html>
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