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><A
NAME="INDEXES-OPCLASS"
>11.6. Operator Classes</A
></H1
><A
NAME="AEN14220"
></A
><P
> An index definition may specify an <I
CLASS="FIRSTTERM"
>operator
class</I
> for each column of an index.
</P><PRE
CLASS="SYNOPSIS"
>CREATE INDEX <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> ON <TT
CLASS="REPLACEABLE"
><I
>table</I
></TT
> (<TT
CLASS="REPLACEABLE"
><I
>column</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>opclass</I
></TT
> [<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 <TT
CLASS="TYPE"
>int4</TT
>
would use the <TT
CLASS="LITERAL"
>int4_ops</TT
> class; this operator
class includes comparison functions for values of type <TT
CLASS="TYPE"
>int4</TT
>.
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
></P
></P><UL
><LI
><P
> The operator classes <TT
CLASS="LITERAL"
>text_pattern_ops</TT
>,
<TT
CLASS="LITERAL"
>varchar_pattern_ops</TT
>,
<TT
CLASS="LITERAL"
>bpchar_pattern_ops</TT
>, and
<TT
CLASS="LITERAL"
>name_pattern_ops</TT
> support B-tree indexes on
the types <TT
CLASS="TYPE"
>text</TT
>, <TT
CLASS="TYPE"
>varchar</TT
>,
<TT
CLASS="TYPE"
>char</TT
>, and <TT
CLASS="TYPE"
>name</TT
>, respectively. The
difference from the ordinary 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 (<TT
CLASS="LITERAL"
>LIKE</TT
> or POSIX
regular expressions) if the server does not use the standard
<SPAN
CLASS="QUOTE"
>"C"</SPAN
> locale. As an example, you might index a
<TT
CLASS="TYPE"
>varchar</TT
> column like this:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX test_index ON test_table (col varchar_pattern_ops);</PRE
><P>
If you do use the C locale, you may instead create an index
with the default operator class, and it will still be useful
for pattern-matching queries. Also note that you should
create an index with the default operator class if you want
queries involving ordinary comparisons to use an index. Such
queries cannot use the
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>xxx</I
></TT
>_pattern_ops</TT
>
operator classes. It is allowed to create multiple
indexes on the same column with different operator classes.
</P
></LI
></UL
><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.oprname 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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