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<title>32.12.User-Defined Operators</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="xoper"></a>32.12.User-Defined Operators</h2></div></div></div>
<a name="id711351"></a><p> Every operator is “<span class="quote">syntactic sugar</span>” for a call to an
underlying function that does the real work; so you must
first create the underlying function before you can create
the operator. However, an operator is <span class="emphasis"><em>not merely</em></span>
syntactic sugar, because it carries additional information
that helps the query planner optimize queries that use the
operator. The next section will be devoted to explaining
that additional information.
</p>
<p> <span class="productname">PostgreSQL</span> supports left unary, right
unary, and binary operators. Operators can be
overloaded;<a name="id711392"></a>
that is, the same operator name can be used for different operators
that have different numbers and types of operands. When a query is
executed, the system determines the operator to call from the
number and types of the provided operands.
</p>
<p> Here is an example of creating an operator for adding two complex
numbers. We assume we've already created the definition of type
<code class="type">complex</code> (see <a href="xtypes.html" title="32.11.User-Defined Types">Section32.11, “User-Defined Types”</a>). First we need a
function that does the work, then we can define the operator:
</p>
<pre class="programlisting">CREATE FUNCTION complex_add(complex, complex)
RETURNS complex
AS '<em class="replaceable"><code>filename</code></em>', 'complex_add'
LANGUAGE C IMMUTABLE STRICT;
CREATE OPERATOR + (
leftarg = complex,
rightarg = complex,
procedure = complex_add,
commutator = +
);</pre>
<p>
</p>
<p> Now we could execute a query like this:
</p>
<pre class="screen">SELECT (a + b) AS c FROM test_complex;
c
-----------------
(5.2,6.05)
(133.42,144.95)</pre>
<p>
</p>
<p> We've shown how to create a binary operator here. To create unary
operators, just omit one of <code class="literal">leftarg</code> (for left unary) or
<code class="literal">rightarg</code> (for right unary). The <code class="literal">procedure</code>
clause and the argument clauses are the only required items in
<code class="command">CREATE OPERATOR</code>. The <code class="literal">commutator</code>
clause shown in the example is an optional hint to the query
optimizer. Further details about <code class="literal">commutator</code> and other
optimizer hints appear in the next section.
</p>
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