Perhaps you are looking for:
- Proposed Database Schema Changes
- Manual:Schema changes — how to build schema change patches for MediaWiki core and its extensions
- Schema changes — process for getting schema changes applied on Wikimedia production. Also contains some generally applicable advice on schema changes and design.
- Manual:Hooks/LoadExtensionSchemaUpdates — a hook allowing extensions to make changes to the database when update.php is used
- the schemas in the Schema: namespace that describe the format of events logged by extension Event logging.