11.4. Indexes and
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
ORDER BY specification to be honored
without a separate sorting step. Of the index types currently
supported by PostgreSQL, only B-tree
can produce sorted output — the other index types return
matching rows in an unspecified, implementation-dependent order.
The planner will consider satisfying an
ORDER BY 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
ORDER BY in combination with
n: an explicit sort will have to process
all the data to identify the first
n rows, but if there is
an index matching the
ORDER BY, the first
rows can be retrieved directly, without scanning the remainder at all.
By default, B-tree indexes store their entries in ascending order
with nulls last. This means that a forward scan of an index on
x produces output satisfying
ORDER BY x
(or more verbosely,
ORDER BY x ASC NULLS LAST). The
index can also be scanned backward, producing output satisfying
ORDER BY x DESC
(or more verbosely,
ORDER BY x DESC NULLS FIRST, since
NULLS FIRST is the default for
ORDER BY DESC).
You can adjust the ordering of a B-tree index by including the
NULLS LAST when creating the index; for example:
CREATE INDEX test2_info_nulls_low ON test2 (info NULLS FIRST); CREATE INDEX test3_desc_index ON test3 (id DESC NULLS LAST);
An index stored in ascending order with nulls first can satisfy
ORDER BY x ASC NULLS FIRST or
ORDER BY x DESC NULLS LAST depending on which direction
it is scanned in.
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
ORDER BY. In single-column indexes
the options are indeed redundant, but in multicolumn indexes they can be
useful. Consider a two-column index on
(x, y): this can
ORDER BY x, y if we scan forward, or
ORDER BY x DESC, y DESC if we scan backward.
But it might be that the application frequently needs to use
ORDER BY x ASC, y DESC. There is no way to get that
ordering from a plain index, but it is possible if the index is defined
(x ASC, y DESC) or
(x DESC, y ASC).
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.