Commit90fdc25

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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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Original file line number	Diff line number	Diff line change
`@@ -704,6 +704,16 @@ Indexes:`
`704`	`704`	`sort high</quote>, in queries that depend on indexes to avoid sorting steps.`
`705`	`705`	`</para>`
`706`	`706`
	`707`	`+ <para>`
	`708`	`+ The regularly system collects statistics on all of a table's`
	`709`	`+ columns. Newly-created non-expression indexes can immediately`
	`710`	`+ use these statistics to determine an index's usefulness.`
	`711`	`+ For new expression indexes, it is necessary to run <link`
	`712`	`+ linkend="sql-analyze"><command>ANALYZE</command></link> or wait for`
	`713`	`+ the <link linkend="autovacuum">autovacuum daemon</link> to analyze`
	`714`	`+ the table to generate statistics about new expression indexes.`
	`715`	`+ </para>`
	`716`	`+`
`707`	`717`	`<para>`
`708`	`718`	`For most index methods, the speed of creating an index is`
`709`	`719`	`dependent on the setting of <xref linkend="guc-maintenance-work-mem"/>.`

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