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<title>11.8.Operator Classes</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="indexes-opclass"></a>11.8.Operator Classes</h2></div></div></div>
<a name="id629345"></a><p> An index definition may specify an <em class="firstterm">operator
class</em> for each column of an index.
</p>
<pre class="synopsis">CREATE INDEX <em class="replaceable"><code>name</code></em> ON <em class="replaceable"><code>table</code></em> (<em class="replaceable"><code>column</code></em> <em class="replaceable"><code>opclass</code></em> [<span class="optional">, ...</span>]);</pre>
<p>
The operator class identifies the operators to be used by the index
for that column. For example, a B-tree index on the type <code class="type">int4</code>
would use the <code class="literal">int4_ops</code> class; this operator
class includes comparison functions for values of type <code class="type">int4</code>.
In practice the default operator class for the column's data type is
usually sufficient. The main point of having operator classes is
that for some data types, there could be more than one meaningful
index behavior. For example, we might want to sort a complex-number data
type either by absolute value or by real part. We could do this by
defining two operator classes for the data type and then selecting
the proper class when making an index.
</p>
<p> There are also some built-in operator classes besides the default ones:
</p>
<div class="itemizedlist"><ul type="disc"><li>
<p> The operator classes <code class="literal">text_pattern_ops</code>,
<code class="literal">varchar_pattern_ops</code>,
<code class="literal">bpchar_pattern_ops</code>, and
<code class="literal">name_pattern_ops</code> support B-tree indexes on
the types <code class="type">text</code>, <code class="type">varchar</code>,
<code class="type">char</code>, and <code class="type">name</code>, respectively. The
difference from the default operator classes is that the values
are compared strictly character by character rather than
according to the locale-specific collation rules. This makes
these operator classes suitable for use by queries involving
pattern matching expressions (<code class="literal">LIKE</code> or POSIX
regular expressions) when the server does not use the standard
“<span class="quote">C</span>” locale. As an example, you might index a
<code class="type">varchar</code> column like this:
</p>
<pre class="programlisting">CREATE INDEX test_index ON test_table (col varchar_pattern_ops);</pre>
<p>
Note that you should also create an index with the default operator
class if you want queries involving ordinary comparisons to use an
index. Such queries cannot use the
<code class="literal"><em class="replaceable"><code>xxx</code></em>_pattern_ops</code>
operator classes. It is allowed to create multiple
indexes on the same column with different operator classes.
If you do use the C locale, you do not need the
<code class="literal"><em class="replaceable"><code>xxx</code></em>_pattern_ops</code>
operator classes, because an index with the default operator class
is usable for pattern-matching queries in the C locale.
</p>
</li></ul></div>
<p>
</p>
<p> The following query shows all defined operator classes:
</p>
<pre class="programlisting">SELECT am.amname AS index_method,
opc.opcname AS opclass_name
FROM pg_am am, pg_opclass opc
WHERE opc.opcamid = am.oid
ORDER BY index_method, opclass_name;</pre>
<p>
It can be extended to show all the operators included in each class:
</p>
<pre class="programlisting">SELECT am.amname AS index_method,
opc.opcname AS opclass_name,
opr.oid::regoperator AS opclass_operator
FROM pg_am am, pg_opclass opc, pg_amop amop, pg_operator opr
WHERE opc.opcamid = am.oid AND
amop.amopclaid = opc.oid AND
amop.amopopr = opr.oid
ORDER BY index_method, opclass_name, opclass_operator;</pre>
<p>
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