Manual:$wgDBmwschema/de-formal

Details
Schema for mediawiki tables. It was originally only used if your backend database was Postgres/MSSQL. Since MediaWiki 1.31 it affects other database types too. This indicates what schema the main mediawiki tables live in, and is roughly equivalent to the way that  is used by MySQL.

The installer of old MediaWiki versions were setting this to  in LocalSettings.php directly, even for MySQL databases, which may cause errors when upgrading. If you get errors when upgrading to MediaWiki 1.31 or later and you're not using Postgres/MSSQL as database backend, remove this setting from LocalSettings.php.