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Diffstat (limited to 'doc/src/sgml/monitoring.sgml')
| -rw-r--r-- | doc/src/sgml/monitoring.sgml | 28 |
1 files changed, 22 insertions, 6 deletions
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml index b1710680705..aaa6586d3a4 100644 --- a/doc/src/sgml/monitoring.sgml +++ b/doc/src/sgml/monitoring.sgml @@ -4234,16 +4234,32 @@ description | Waiting for a newly initialized WAL file to reach durable storage <note> <para> - Queries that use certain <acronym>SQL</acronym> constructs to search for - rows matching any value out of a list or array of multiple scalar values - (see <xref linkend="functions-comparisons"/>) perform multiple - <quote>primitive</quote> index scans (up to one primitive scan per scalar - value) during query execution. Each internal primitive index scan - increments <structname>pg_stat_all_indexes</structname>.<structfield>idx_scan</structfield>, + Index scans may sometimes perform multiple index searches per execution. + Each index search increments <structname>pg_stat_all_indexes</structname>.<structfield>idx_scan</structfield>, so it's possible for the count of index scans to significantly exceed the total number of index scan executor node executions. </para> + <para> + This can happen with queries that use certain <acronym>SQL</acronym> + constructs to search for rows matching any value out of a list or array of + multiple scalar values (see <xref linkend="functions-comparisons"/>). It + can also happen to queries with a + <literal><replaceable>column_name</replaceable> = + <replaceable>value1</replaceable> OR + <replaceable>column_name</replaceable> = + <replaceable>value2</replaceable> ...</literal> construct, though only + when the optimizer transforms the construct into an equivalent + multi-valued array representation. + </para> </note> + <tip> + <para> + <command>EXPLAIN ANALYZE</command> outputs the total number of index + searches performed by each index scan node. See + <xref linkend="using-explain-analyze"/> for an example demonstrating how + this works. + </para> + </tip> </sect2> |