Seeing as T191231: RFC: Abstract schemas and schema changes has been approved, rather than doing tasks like T244898: Notifications ('Echo') should work with postgres, T244899: CentralAuth should work with postgres and T199544: Make AbuseFilter work on PostgreSQL and SQLite (epic), it seems prudent to convert the current schemas to the abstract syntax and use that to generate the PG schema, rather than building a schema manually ourselves
The TLDR guide for this:
- Write a tables.json for the schema (see maintenance/tables.json in mw core)
- Run maintenance/generateSchemaSql.php on the tables.json you created to generate the sql files
- git add the new files. Wire them in as normal in your LoadExtensionSchemaUpdates subscriber
More information can be seen at https://www.mediawiki.org/wiki/Manual:Schema_changes