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><H1
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><A
NAME="INDEXES-OPCLASS"
>11.9. Operator Classes and Operator Families</A
></H1
><P
> An index definition can 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"
><TT
CLASS="REPLACEABLE"
><I
>sort options</I
></TT
></SPAN
>] [<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 reason for 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. The operator class determines
the basic sort ordering (which can then be modified by adding sort options
<TT
CLASS="LITERAL"
>COLLATE</TT
>,
<TT
CLASS="LITERAL"
>ASC</TT
>/<TT
CLASS="LITERAL"
>DESC</TT
> and/or
<TT
CLASS="LITERAL"
>NULLS FIRST</TT
>/<TT
CLASS="LITERAL"
>NULLS LAST</TT
>).
</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
>, and
<TT
CLASS="LITERAL"
>bpchar_pattern_ops</TT
> support B-tree indexes on
the types <TT
CLASS="TYPE"
>text</TT
>, <TT
CLASS="TYPE"
>varchar</TT
>, and
<TT
CLASS="TYPE"
>char</TT
> 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 (<TT
CLASS="LITERAL"
>LIKE</TT
> or POSIX
regular expressions) when the database 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>
Note that you should also create an index with the default operator
class if you want queries involving ordinary <TT
CLASS="LITERAL"
><</TT
>,
<TT
CLASS="LITERAL"
><=</TT
>, <TT
CLASS="LITERAL"
>></TT
>, or <TT
CLASS="LITERAL"
>>=</TT
> 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. (Ordinary equality comparisons can use these
operator classes, however.) It is possible to create multiple
indexes on the same column with different operator classes.
If you do use the C locale, you do not need the
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>xxx</I
></TT
>_pattern_ops</TT
>
operator classes, because an index with the default operator class
is usable for pattern-matching queries in the C locale.
</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.opcmethod = am.oid
ORDER BY index_method, opclass_name;</PRE
><P>
</P
><P
> An operator class is actually just a subset of a larger structure called an
<I
CLASS="FIRSTTERM"
>operator family</I
>. In cases where several data types have
similar behaviors, it is frequently useful to define cross-data-type
operators and allow these to work with indexes. To do this, the operator
classes for each of the types must be grouped into the same operator
family. The cross-type operators are members of the family, but are not
associated with any single class within the family.
</P
><P
> This query shows all defined operator families and all
the operators included in each family:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT am.amname AS index_method,
opf.opfname AS opfamily_name,
amop.amopopr::regoperator AS opfamily_operator
FROM pg_am am, pg_opfamily opf, pg_amop amop
WHERE opf.opfmethod = am.oid AND
amop.amopfamily = opf.oid
ORDER BY index_method, opfamily_name, opfamily_operator;</PRE
><P>
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