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docs: mention the optimizer can increase the index usage count 

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>
<para> <para>
Indexes can be used via either simple index scans or <quote>bitmap</> Indexes can be used by simple index scans, <quote>bitmap</> index scans,
index scans. In a bitmap scan and the optimizer. In a bitmap scan
the output of several indexes can be combined via AND or OR rules, the output of several indexes can be combined via AND or OR rules,
so it is difficult to associate individual heap row fetches so it is difficult to associate individual heap row fetches
with specific indexes when a bitmap scan is used. Therefore, a bitmap 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</> <structname>pg_stat_all_tables</>.<structfield>idx_tup_fetch</>
count for the table, but it does not affect count for the table, but it does not affect
<structname>pg_stat_all_indexes</>.<structfield>idx_tup_fetch</>. <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> </para>
<note> <note>