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>Chapter 11. Indexes</TD
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><H1
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><A
NAME="INDEXES-TYPES"
>11.2. Index Types</A
></H1
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides several index types:
B-tree, Hash, GiST and GIN. Each index type uses a different
algorithm that is best suited to different types of queries.
By default, the <TT
CLASS="COMMAND"
>CREATE INDEX</TT
> command will create a
B-tree index, which fits the most common situations.
</P
><P
> <A
NAME="AEN17524"
></A
>
<A
NAME="AEN17527"
></A
>
B-trees can handle equality and range queries on data that can be sorted
into some ordering.
In particular, the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> query planner
will consider using a B-tree index whenever an indexed column is
involved in a comparison using one of these operators:
<P
></P
><TABLE
BORDER="0"
><TBODY
><TR
><TD
><TT
CLASS="LITERAL"
><</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
><=</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>=</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>>=</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>></TT
></TD
></TR
></TBODY
></TABLE
><P
></P
>
Constructs equivalent to combinations of these operators, such as
<TT
CLASS="LITERAL"
>BETWEEN</TT
> and <TT
CLASS="LITERAL"
>IN</TT
>, can also be implemented with
a B-tree index search. Also, an <TT
CLASS="LITERAL"
>IS NULL</TT
> condition on
an index column can be used with a B-tree index.
</P
><P
> The optimizer can also use a B-tree index for queries involving the
pattern matching operators <TT
CLASS="LITERAL"
>LIKE</TT
> and <TT
CLASS="LITERAL"
>~</TT
>
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>if</I
></SPAN
> the pattern is a constant and is anchored to
the beginning of the string — for example, <TT
CLASS="LITERAL"
>col LIKE
'foo%'</TT
> or <TT
CLASS="LITERAL"
>col ~ '^foo'</TT
>, but not
<TT
CLASS="LITERAL"
>col LIKE '%bar'</TT
>. However, if your server does not
use the C locale you will need to create the index with a special
operator class to support indexing of pattern-matching queries. See
<A
HREF="indexes-opclass.html"
>Section 11.9</A
> below. It is also possible to use
B-tree indexes for <TT
CLASS="LITERAL"
>ILIKE</TT
> and
<TT
CLASS="LITERAL"
>~*</TT
>, but only if the pattern starts with
non-alphabetic characters, i.e. characters that are not affected by
upper/lower case conversion.
</P
><P
> <A
NAME="AEN17556"
></A
>
<A
NAME="AEN17559"
></A
>
Hash indexes can only handle simple equality comparisons.
The query planner will consider using a hash index whenever an
indexed column is involved in a comparison using the
<TT
CLASS="LITERAL"
>=</TT
> operator. (But hash indexes do not support
<TT
CLASS="LITERAL"
>IS NULL</TT
> searches.)
The following command is used to create a hash index:
</P><PRE
CLASS="SYNOPSIS"
>CREATE INDEX <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> ON <TT
CLASS="REPLACEABLE"
><I
>table</I
></TT
> USING hash (<TT
CLASS="REPLACEABLE"
><I
>column</I
></TT
>);</PRE
><P>
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Testing has shown <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s hash
indexes to perform no better than B-tree indexes, and the
index size and build time for hash indexes is much worse.
Furthermore, hash index operations are not presently WAL-logged,
so hash indexes might need to be rebuilt with <TT
CLASS="COMMAND"
>REINDEX</TT
>
after a database crash.
For these reasons, hash index use is presently discouraged.
</P
></BLOCKQUOTE
></DIV
><P
> <A
NAME="AEN17573"
></A
>
<A
NAME="AEN17576"
></A
>
GiST indexes are not a single kind of index, but rather an infrastructure
within which many different indexing strategies can be implemented.
Accordingly, the particular operators with which a GiST index can be
used vary depending on the indexing strategy (the <I
CLASS="FIRSTTERM"
>operator
class</I
>). As an example, the standard distribution of
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> includes GiST operator classes
for several two-dimensional geometric data types, which support indexed
queries using these operators:
<P
></P
><TABLE
BORDER="0"
><TBODY
><TR
><TD
><TT
CLASS="LITERAL"
><<</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>&<</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>&></TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>>></TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
><<|</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>&<|</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>|&></TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>|>></TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>@></TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
><@</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>~=</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>&&</TT
></TD
></TR
></TBODY
></TABLE
><P
></P
>
(See <A
HREF="functions-geometry.html"
>Section 9.11</A
> for the meaning of
these operators.)
Many other GiST operator
classes are available in the <TT
CLASS="LITERAL"
>contrib</TT
> collection or as separate
projects. For more information see <A
HREF="gist.html"
>Chapter 51</A
>.
</P
><P
> <A
NAME="AEN17610"
></A
>
<A
NAME="AEN17613"
></A
>
GIN indexes are inverted indexes which can handle values that contain more
than one key, arrays for example. Like GiST, GIN can support
many different user-defined indexing strategies and the particular
operators with which a GIN index can be used vary depending on the
indexing strategy.
As an example, the standard distribution of
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> includes GIN operator classes
for one-dimensional arrays, which support indexed
queries using these operators:
<P
></P
><TABLE
BORDER="0"
><TBODY
><TR
><TD
><TT
CLASS="LITERAL"
><@</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>@></TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>=</TT
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
>&&</TT
></TD
></TR
></TBODY
></TABLE
><P
></P
>
(See <A
HREF="functions-array.html"
>Section 9.17</A
> for the meaning of
these operators.)
Many other GIN operator
classes are available in the <TT
CLASS="LITERAL"
>contrib</TT
> collection or as separate
projects. For more information see <A
HREF="gin.html"
>Chapter 52</A
>.
</P
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