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><A
NAME="INDEXES-ORDERING"
>11.4. Indexes and <TT
CLASS="LITERAL"
>ORDER BY</TT
></A
></H1
><P
> In addition to simply finding the rows to be returned by a query,
an index may be able to deliver them in a specific sorted order.
This allows a query's <TT
CLASS="LITERAL"
>ORDER BY</TT
> specification to be honored
without a separate sorting step. Of the index types currently
supported by <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, only B-tree
can produce sorted output — the other index types return
matching rows in an unspecified, implementation-dependent order.
</P
><P
> The planner will consider satisfying an <TT
CLASS="LITERAL"
>ORDER BY</TT
> specification
either by scanning an available index that matches the specification,
or by scanning the table in physical order and doing an explicit
sort. For a query that requires scanning a large fraction of the
table, an explicit sort is likely to be faster than using an index
because it requires
less disk I/O due to following a sequential access pattern. Indexes are
more useful when only a few rows need be fetched. An important
special case is <TT
CLASS="LITERAL"
>ORDER BY</TT
> in combination with
<TT
CLASS="LITERAL"
>LIMIT</TT
> <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>: an explicit sort will have to process
all the data to identify the first <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> rows, but if there is
an index matching the <TT
CLASS="LITERAL"
>ORDER BY</TT
>, the first <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>
rows can be retrieved directly, without scanning the remainder at all.
</P
><P
> By default, B-tree indexes store their entries in ascending order
with nulls last. This means that a forward scan of an index on
column <TT
CLASS="LITERAL"
>x</TT
> produces output satisfying <TT
CLASS="LITERAL"
>ORDER BY x</TT
>
(or more verbosely, <TT
CLASS="LITERAL"
>ORDER BY x ASC NULLS LAST</TT
>). The
index can also be scanned backward, producing output satisfying
<TT
CLASS="LITERAL"
>ORDER BY x DESC</TT
>
(or more verbosely, <TT
CLASS="LITERAL"
>ORDER BY x DESC NULLS FIRST</TT
>, since
<TT
CLASS="LITERAL"
>NULLS FIRST</TT
> is the default for <TT
CLASS="LITERAL"
>ORDER BY DESC</TT
>).
</P
><P
> You can adjust the ordering of a B-tree index by including the
options <TT
CLASS="LITERAL"
>ASC</TT
>, <TT
CLASS="LITERAL"
>DESC</TT
>, <TT
CLASS="LITERAL"
>NULLS FIRST</TT
>,
and/or <TT
CLASS="LITERAL"
>NULLS LAST</TT
> when creating the index; for example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX test2_info_nulls_low ON test2 (info NULLS FIRST);
CREATE INDEX test3_desc_index ON test3 (id DESC NULLS LAST);</PRE
><P>
An index stored in ascending order with nulls first can satisfy
either <TT
CLASS="LITERAL"
>ORDER BY x ASC NULLS FIRST</TT
> or
<TT
CLASS="LITERAL"
>ORDER BY x DESC NULLS LAST</TT
> depending on which direction
it is scanned in.
</P
><P
> You might wonder why bother providing all four options, when two
options together with the possibility of backward scan would cover
all the variants of <TT
CLASS="LITERAL"
>ORDER BY</TT
>. In single-column indexes
the options are indeed redundant, but in multicolumn indexes they can be
useful. Consider a two-column index on <TT
CLASS="LITERAL"
>(x, y)</TT
>: this can
satisfy <TT
CLASS="LITERAL"
>ORDER BY x, y</TT
> if we scan forward, or
<TT
CLASS="LITERAL"
>ORDER BY x DESC, y DESC</TT
> if we scan backward.
But it might be that the application frequently needs to use
<TT
CLASS="LITERAL"
>ORDER BY x ASC, y DESC</TT
>. There is no way to get that
ordering from a plain index, but it is possible if the index is defined
as <TT
CLASS="LITERAL"
>(x ASC, y DESC)</TT
> or <TT
CLASS="LITERAL"
>(x DESC, y ASC)</TT
>.
</P
><P
> Obviously, indexes with non-default sort orderings are a fairly
specialized feature, but sometimes they can produce tremendous
speedups for certain queries. Whether it's worth maintaining such an
index depends on how often you use queries that require a special
sort ordering.
</P
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