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Diffstat (limited to 'doc/src/sgml/indices.sgml')
| -rw-r--r-- | doc/src/sgml/indices.sgml | 53 |
1 files changed, 33 insertions, 20 deletions
diff --git a/doc/src/sgml/indices.sgml b/doc/src/sgml/indices.sgml index a1c1c9735b1..fc268389e85 100644 --- a/doc/src/sgml/indices.sgml +++ b/doc/src/sgml/indices.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/indices.sgml,v 1.52 2005/09/12 19:17:45 tgl Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/indices.sgml,v 1.53 2005/10/21 01:41:28 tgl Exp $ --> <chapter id="indexes"> <title id="indexes-title">Indexes</title> @@ -206,14 +206,6 @@ CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> <synopsis> CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> USING hash (<replaceable>column</replaceable>); </synopsis> - <note> - <para> - Testing has shown <productname>PostgreSQL</productname>'s hash - indexes to perform no better than B-tree indexes, and the - index size and build time for hash indexes is much worse. For - these reasons, hash index use is presently discouraged. - </para> - </note> </para> <para> @@ -226,15 +218,33 @@ CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> equivalent to the R-tree operator classes, and many other GiST operator classes are available in the <literal>contrib</> collection or as separate projects. For more information see <xref linkend="GiST">. - <note> - <para> - It is likely that the R-tree index type will be retired in a future - release, as GiST indexes appear to do everything R-trees can do with - similar or better performance. Users are encouraged to migrate - applications that use R-tree indexes to GiST indexes. - </para> - </note> </para> + + <note> + <para> + Testing has shown <productname>PostgreSQL</productname>'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 may need to be rebuilt with <command>REINDEX</> + after a database crash. + For these reasons, hash index use is presently discouraged. + </para> + + <para> + Similarly, R-tree indexes do not seem to have any performance + advantages compared to the equivalent operations of GiST indexes. + Like hash indexes, they are not WAL-logged and may need + <command>REINDEX</>ing after a database crash. + </para> + + <para> + While the problems with hash indexes may be fixed eventually, + it is likely that the R-tree index type will be retired in a future + release. Users are encouraged to migrate applications that use R-tree + indexes to GiST indexes. + </para> + </note> </sect1> @@ -300,9 +310,12 @@ CREATE INDEX test2_mm_idx ON test2 (major, minor); <para> A multicolumn GiST index can only be used when there is a query condition - on its leading column. As with B-trees, conditions on additional columns - restrict the entries returned by the index, but do not in themselves aid - the index search. + on its leading column. Conditions on additional columns restrict the + entries returned by the index, but the condition on the first column is the + most important one for determining how much of the index needs to be + scanned. A GiST index will be relatively ineffective if its first column + has only a few distinct values, even if there are many distinct values in + additional columns. </para> <para> |