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+# Database Review Guidelines
+
+This page is specific to database reviews. Please refer to our
+[code review guide](code_review.md) for broader advice and best
+practices for code review in general.
+
+## General process
+
+A database review is required for:
+
+- Changes that touch the database schema or perform data migrations,
+  including files in:
+  - `db/`
+  - `lib/gitlab/background_migration/`
+- Changes to the database tooling, e.g.:
+  - migration or ActiveRecord helpers in `lib/gitlab/database/`
+  - load balancing
+- Changes that produce SQL queries that are beyond the obvious. It is
+  generally up to the author of a merge request to decide whether or
+  not complex queries are being introduced and if they require a
+  database review.
+
+A database reviewer is expected to look out for obviously complex
+queries in the change and review those closer. If the author does not
+point out specific queries for review and there are no obviously
+complex queries, it is enough to concentrate on reviewing the
+migration only.
+
+It is preferable to review queries in SQL form and generally accepted
+to ask the author to translate any ActiveRecord queries in SQL form
+for review.
+
+### Roles and process
+
+A Merge Request author's role is to:
+
+- Decide whether a database review is needed.
+- If database review is needed, add the ~database label.
+- Use the [database changes](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.gitlab/merge_request_templates/Database%20changes.md)
+  merge request template, or include the appropriate items in the MR description.
+
+A database **reviewer**'s role is to:
+
+- Perform a first-pass review on the MR and suggest improvements to the author.
+- Once satisfied, relabel the MR with ~"database::reviewed", approve it, and
+  reassign MR to the database **maintainer** suggested by Reviewer
+  Roulette.
+
+A database **maintainer**'s role is to:
+
+- Perform the final database review on the MR.
+- Discuss further improvements or other relevant changes with the
+  database reviewer and the MR author.
+- Finally approve the MR and relabel the MR with ~"database::approved"
+- Merge the MR if no other approvals are pending or pass it on to
+  other maintainers as required (frontend, backend, docs).
+
+### Distributing review workload
+
+Review workload is distributed using [reviewer roulette](code_review.md#reviewer-roulette)
+([example](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25181#note_147551725)).
+The MR author should then co-assign the suggested database
+**reviewer**. When they give their sign-off, they will hand over to
+the suggested database **maintainer**.
+
+If reviewer roulette didn't suggest a database reviewer & maintainer,
+make sure you have applied the ~database label and rerun the
+`danger-review` CI job, or pick someone from the
+[`@gl-database` team](https://gitlab.com/groups/gl-database/-/group_members).
+
+### How to review for database
+
+- Check migrations
+  - Review relational modeling and design choices
+  - Review migrations follow [database migration style guide](migration_style_guide.md),
+    for example
+    - [Check ordering of columns](ordering_table_columns.md)
+    - [Check indexes are present for foreign keys](migration_style_guide.md#adding-foreign-key-constraints)
+  - Ensure that migrations execute in a transaction or only contain
+    concurrent index/foreign key helpers (with transactions disabled)
+  - Check consistency with `db/schema.rb` and that migrations are [reversible](migration_style_guide.md#reversibility)
+  - For data migrations, establish a time estimate for execution
+- Check post deploy migration
+  - Make sure we can expect post deploy migrations to finish within 1 hour max.
+- Check background migrations
+  - Review queries (for example, make sure batch sizes are fine)
+  - Establish a time estimate for execution
+- Query performance
+  - Check for any obviously complex queries and queries the author specifically
+    points out for review (if any)
+  - If not present yet, ask the author to provide SQL queries and query plans
+    (e.g. by using [chatops](understanding_explain_plans.md#chatops) or direct
+    database access)
+  - For given queries, review parameters regarding data distribution
+  - [Check query plans](understanding_explain_plans.md) and suggest improvements
+    to queries (changing the query, schema or adding indexes and similar)
+  - General guideline is for queries to come in below 100ms execution time
+  - If queries rely on prior migrations that are not present yet on production
+    (eg indexes, columns), you can use a [one-off instance from the restore
+    pipeline](https://ops.gitlab.net/gitlab-com/gl-infra/gitlab-restore/postgres-gprd)
+    in order to establish a proper testing environment.