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 $wgDBprefix is used by MySQL.
The installer of old MediaWiki versions were setting this to 'mediawiki' 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.
Do not change $wgDBmwschema after Setup.php has been included. Note:Setup.php initializes various paths, includes some common files, starts sessions, and performs various other setup tasks for a request. It is not related to the installation or setup of MediaWiki.
If you configure $wgDBservers or $wgLBFactoryConf, you still need to define $wgDBmwschema. The combination of $wgDBname, $wgDBmwschema, and $wgDBprefix define the current wiki's database domain. This is also called the local database domain. The 'dbname' and 'tablePrefix' settings in $wgDBservers and $wgLBFactoryConf only define the default DB/prefix when a connection is requested with DatabaseDomain::newUnspecified()->getId() (the empty string) as the database domain, e.g. wfGetDB( DB_REPLICA, , '' ). Note that functions calls like wfGetDB( DB_REPLICA ), which simply omit the database domain, are assumed to refer to the local database domain.
If you change this setting, you should probably create a new user (and change $wgDBuser as well), because the configuration script sets the user's default path to include the schema.