Manual:$wgSearchType

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Database settings: $wgSearchType
Type of DB search used.
Introduced in version:	1.4.0
Removed in version:	still in use
Allowed Values:	(string) or null. See below for valid values.
Default Value:	null "MyISAM" prior to 1.5.0.

Other settings: Alphabetical | By Function

[edit] Details

Leave as null to select the default search engine for the selected database type (e.g. SearchMySQL, SearchPostgres or SearchOracle), or set to a class name to override to a custom search engine.