134 lines
6.9 KiB
Markdown
134 lines
6.9 KiB
Markdown
# Database Review Guidelines
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This page is specific to database reviews. Please refer to our
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[code review guide](code_review.md) for broader advice and best
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practices for code review in general.
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## General process
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A database review is required for:
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- Changes that touch the database schema or perform data migrations,
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including files in:
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- `db/`
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- `lib/gitlab/background_migration/`
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- Changes to the database tooling, e.g.:
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- migration or ActiveRecord helpers in `lib/gitlab/database/`
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- load balancing
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- Changes that produce SQL queries that are beyond the obvious. It is
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generally up to the author of a merge request to decide whether or
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not complex queries are being introduced and if they require a
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database review.
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A database reviewer is expected to look out for obviously complex
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queries in the change and review those closer. If the author does not
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point out specific queries for review and there are no obviously
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complex queries, it is enough to concentrate on reviewing the
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migration only.
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It is preferable to review queries in SQL form and generally accepted
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to ask the author to translate any ActiveRecord queries in SQL form
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for review.
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### Roles and process
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A Merge Request author's role is to:
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- Decide whether a database review is needed.
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- If database review is needed, add the ~database label.
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- Use the [database changes](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.gitlab/merge_request_templates/Database%20changes.md)
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merge request template, or include the appropriate items in the MR description.
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A database **reviewer**'s role is to:
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- Perform a first-pass review on the MR and suggest improvements to the author.
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- Once satisfied, relabel the MR with ~"database::reviewed", approve it, and
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reassign MR to the database **maintainer** suggested by Reviewer
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Roulette.
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A database **maintainer**'s role is to:
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- Perform the final database review on the MR.
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- Discuss further improvements or other relevant changes with the
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database reviewer and the MR author.
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- Finally approve the MR and relabel the MR with ~"database::approved"
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- Merge the MR if no other approvals are pending or pass it on to
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other maintainers as required (frontend, backend, docs).
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### Distributing review workload
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Review workload is distributed using [reviewer roulette](code_review.md#reviewer-roulette)
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([example](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25181#note_147551725)).
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The MR author should then co-assign the suggested database
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**reviewer**. When they give their sign-off, they will hand over to
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the suggested database **maintainer**.
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If reviewer roulette didn't suggest a database reviewer & maintainer,
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make sure you have applied the ~database label and rerun the
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`danger-review` CI job, or pick someone from the
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[`@gl-database` team](https://gitlab.com/groups/gl-database/-/group_members).
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### How to prepare for speedy database reviews
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In order to make reviewing easier and therefore faster, please consider preparing a comment
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and details for a database reviewer:
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- Provide queries in SQL form rather than ActiveRecord.
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- Format any queries with a SQL query formatter, for example with [sqlformat.darold.net](http://sqlformat.darold.net).
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- Consider providing query plans via a link to [explain.depesz.com](https://explain.depesz.com) or another tool instead of textual form.
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- For query changes, it is best to provide the SQL query along with a plan *before* and *after* the change. This helps to spot differences quickly.
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- When providing query plans, make sure to use good parameter values, so that the query executed is a good example and also hits enough data. Usually, the `gitlab-org` namespace (`namespace_id = 9970`) and the `gitlab-org/gitlab-ce` project (`project_id = 13083`) provides enough data to serve as a good example.
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### How to review for database
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- Check migrations
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- Review relational modeling and design choices
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- Review migrations follow [database migration style guide](migration_style_guide.md),
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for example
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- [Check ordering of columns](ordering_table_columns.md)
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- [Check indexes are present for foreign keys](migration_style_guide.md#adding-foreign-key-constraints)
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- Ensure that migrations execute in a transaction or only contain
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concurrent index/foreign key helpers (with transactions disabled)
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- Check consistency with `db/schema.rb` and that migrations are [reversible](migration_style_guide.md#reversibility)
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- Check queries timing (If any): Queries executed in a migration
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need to fit comfortable within `15s` - preferably much less than that - on GitLab.com.
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- Check [background migrations](background_migrations.md):
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- For data migrations, establish a time estimate for execution
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- They should only be used when migrating data in larger tables.
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- If a single `update` is below than `1s` the query can be placed
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directly in a regular migration (inside `db/migrate`).
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- Review queries (for example, make sure batch sizes are fine)
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- Establish a time estimate for execution
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- Because execution time can be longer than for a regular migration,
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it's suggested to treat background migrations as post migrations:
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place them in `db/post_migrate` instead of `db/migrate`. Keep in mind
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that post migrations are executed post-deployment in production.
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- Check [timing guidelines for migrations](#timing-guidelines-for-migrations)
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- Query performance
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- Check for any obviously complex queries and queries the author specifically
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points out for review (if any)
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- If not present yet, ask the author to provide SQL queries and query plans
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(e.g. by using [chatops](understanding_explain_plans.md#chatops) or direct
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database access)
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- For given queries, review parameters regarding data distribution
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- [Check query plans](understanding_explain_plans.md) and suggest improvements
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to queries (changing the query, schema or adding indexes and similar)
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- General guideline is for queries to come in below 100ms execution time
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- If queries rely on prior migrations that are not present yet on production
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(eg indexes, columns), you can use a [one-off instance from the restore
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pipeline](https://ops.gitlab.net/gitlab-com/gl-infra/gitlab-restore/postgres-gprd)
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in order to establish a proper testing environment.
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### Timing guidelines for migrations
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In general, migrations for a single deploy shouldn't take longer than
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1 hour for GitLab.com. The following guidelines are not hard rules, they were
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estimated to keep migration timing to a minimum.
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NOTE: **Note:** Keep in mind that all runtimes should be measured against GitLab.com.
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| Migration Type | Execution Time Recommended | Notes |
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|----|----|---|
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| Regular migrations on `db/migrate` | `3 minutes` | A valid exception are index creation as this can take a long time. |
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| Post migrations on `db/post_migrate` | `10 minutes` | |
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| Background migrations | --- | Since these are suitable for larger tables, it's not possible to set a precise timing guideline, however, any query must stay well below `10s` of execution time. |
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