mirror of https://github.com/zulip/zulip.git
26 lines
1.1 KiB
Markdown
26 lines
1.1 KiB
Markdown
# Schema Migrations
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Zulip uses the [standard Django system for doing schema
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migrations](https://docs.djangoproject.com/en/1.8/topics/migrations/).
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There is some example usage in the [new feature
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tutorial](new-feature-tutorial.html).
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This page documents some important issues related to writing schema
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migrations.
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* **Large tables**: For large tables like Message and UserMessage, you
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want to take precautions when adding columns to the table,
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performing data backfills, or building indexes. We have a
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`zerver/lib/migrate.py` library to help with adding columns and
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backfilling data. For building indexes on these tables, we should do
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this using SQL with postgres's CONCURRENTLY keyword.
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* **Numbering conflicts across branches**: If you've done your schema
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change in a branch, and meanwhile another schema change has taken
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place, Django will now have two migrations with the same number. To
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fix this, you need to renumber your migration(s), fix up
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the "dependencies" entries in your migration(s), and rewrite your
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git history as needed. There is a tutorial
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[here](migration-renumbering.html) that walks you though that
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process.
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