mirror of https://github.com/zulip/zulip.git
24 lines
1.0 KiB
Markdown
24 lines
1.0 KiB
Markdown
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# 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 Zulip new feature tutorial on
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readthedocs.
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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 can just rename the file, as long as no other
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migrations depend on it (in which case you also need to update the
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dependencies).
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