Manual:DatabaseUpdater.php

DatabaseUpdater is the class for handling database updates (e.g. adding, modifying, and dropping database tables, fields, and indexes). ,,  ,  , and  extend this class, and all five of those files (as well as the  files for the various database types) potentially need to be revised when a core schema change is made. Extensions usually change the schema by means of hook functions. Either way, SQL patch file(s) need to be created.