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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="43.11.1. Variable Substitution">Section 43.11.1</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="43.11.2. Plan Caching">Section 43.11.2</a>.
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