zulip/docs/schema-migrations.md

Schema Migrations

Zulip uses the standard Django system for doing schema migrations. There is some example usage in the Zulip new feature tutorial on readthedocs.

This page documents some important issues related to writing schema migrations.

• Large tables: For large tables like Message and UserMessage, you want to take precautions when adding columns to the table, performing data backfills, or building indexes. We have a zerver/lib/migrate.py library to help with adding columns and backfilling data. For building indexes on these tables, we should do this using SQL with postgres's CONCURRENTLY keyword.

• Numbering conflicts across branches: If you've done your schema change in a branch, and meanwhile another schema change has taken place, Django will now have two migrations with the same number. To fix this, you can just rename the file, as long as no other migrations depend on it (in which case you also need to update the dependencies).