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Clarify db index docs #13561

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May 1, 2025
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Clarify db index docs #13561

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5 changes: 3 additions & 2 deletions develop-docs/backend/application-domains/database-migrations/index.mdx
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Expand Up @@ -135,11 +135,12 @@ generally prefer that, since it averages the load out over a longer period of ti

### Indexes

We prefer to create indexes on large existing tables with `CREATE INDEX CONCURRENTLY`. Our migration framework will do this automatically when creating
a new index. Note that when `CONCURRENTLY` is used we can't run the migration in a transaction, so it's important to use `atomic = False` to run these.
These are automatically handled by the migration framework, you can just create them on the Django model and generate the migration.

When adding indexes to large tables you should use a `is_post_deployment` migration as creating the index could take longer than the migration statement timeout of 5s.

Note: These are created using `CONCURRENTLY`, so it's important to not set `atomic = True` for migrations that contain indexes. This is disabled by default.
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I think it'd be a little clearer if the atomic = True was part of the first paragraph, like "these are things we should be doing when we want to generate a new index"

Then in the note, it can explain that it's because we use concurrently.

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Generally no action is required about atomic. It's false by default, so it's just a side note in case someone tries to enable it for some reason.


### Deleting columns

This is complicated due to our deploy process. When we deploy, we run migrations, and then push out the application code, which takes a while. This means that if we just delete a column or model, then code in sentry will be looking for those columns/tables and erroring until the deploy completes. In some cases, this can mean Sentry is hard down until the deploy is finished.
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