As the title.
Dunno if this is caused by recent refactorings.. Should mysql be even using that variable?
https://www.mediawiki.org/wiki/Manual:$wgDBmwschema
Schema for mediawiki tables. Currently only used if your backend database is Postgres. This indicates what schema the main mediawiki tables live in, and is roughly equivalent to the way that $wgDBprefix is used by MySQL.