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docs: mention the optimizer can increase the index usage count

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Report by Marko Tiikkaja
This commit is contained in:
Bruce Momjian 2015-03-19 22:38:12 -04:00
parent 28beb69f8b
commit 7e9ed623d9
1 changed files with 5 additions and 2 deletions
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doc/src/sgml/monitoring.sgml
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@ -1382,8 +1382,8 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
</para>
<para>
Indexes can be used via either simple index scans or <quote>bitmap</>
index scans. In a bitmap scan
Indexes can be used by simple index scans, <quote>bitmap</> index scans,
and the optimizer. In a bitmap scan
the output of several indexes can be combined via AND or OR rules,
so it is difficult to associate individual heap row fetches
with specific indexes when a bitmap scan is used. Therefore, a bitmap
@ -1393,6 +1393,9 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<structname>pg_stat_all_tables</>.<structfield>idx_tup_fetch</>
count for the table, but it does not affect
<structname>pg_stat_all_indexes</>.<structfield>idx_tup_fetch</>.
The optimizer also accesses indexes to check for supplied constants
whose values are outside the recorded range of the optimizer statistics
because the optimizer statistics might be stale.
</para>
<note>