Commit3370207

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docs: mention that expression indexes need analyze

Expression indexes can't benefit from pre-computed statistics oncolumns.Reported-by: Nikolay SamokhvalovDiscussion:https://postgr.es/m/CANNMO++5rw9RDA=p40iMVbMNPaW6O=S0AFzTU=KpYHRpCd1voA@mail.gmail.comAuthor: Nikolay Samokhvalov, modifiedBackpatch-through: 9.5

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`@@ -745,6 +745,16 @@ Indexes:`
`745`	`745`	`sort high</quote>, in queries that depend on indexes to avoid sorting steps.`
`746`	`746`	`</para>`
`747`	`747`
	`748`	`+ <para>`
	`749`	`+ The regularly system collects statistics on all of a table's`
	`750`	`+ columns. Newly-created non-expression indexes can immediately`
	`751`	`+ use these statistics to determine an index's usefulness.`
	`752`	`+ For new expression indexes, it is necessary to run <link`
	`753`	`+ linkend="sql-analyze"><command>ANALYZE</command></link> or wait for`
	`754`	`+ the <link linkend="autovacuum">autovacuum daemon</link> to analyze`
	`755`	`+ the table to generate statistics about new expression indexes.`
	`756`	`+ </para>`
	`757`	`+`
`748`	`758`	`<para>`
`749`	`759`	`For most index methods, the speed of creating an index is`
`750`	`760`	`dependent on the setting of <xref linkend="guc-maintenance-work-mem"/>.`

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