gitlab-org--gitlab-foss/doc/development/database_review.md

Database Review Guidelines

This page is specific to database reviews. Please refer to our code review guide 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 merge request template, or include the appropriate items in the MR description.
• Prepare the merge request for a database review.

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 (example). 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.

How to prepare the merge request for a database review

In order to make reviewing easier and therefore faster, please consider preparing a comment and details for a database reviewer:

• Provide queries in SQL form rather than ActiveRecord.
• Format any queries with a SQL query formatter, for example with sqlformat.darold.net.
• Consider providing query plans via a link to explain.depesz.com or another tool instead of textual form.
• 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.
• 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-foss project (project_id = 13083) provides enough data to serve as a good example.

How to review for database

• Check migrations
  • Review relational modeling and design choices
  • Review migrations follow database migration style guide, for example
    • Check ordering of columns
    • Check indexes are present for foreign keys
  • 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
  • Check queries timing (If any): Queries executed in a migration need to fit comfortably within 15s - preferably much less than that - on GitLab.com.
• Check background migrations:
  • Establish a time estimate for execution
  • They should only be used when migrating data in larger tables.
    • If a single update is below than 1s the query can be placed directly in a regular migration (inside db/migrate).
  • Review queries (for example, make sure batch sizes are fine)
  • Because execution time can be longer than for a regular migration, it's suggested to treat background migrations as post migrations: place them in db/post_migrate instead of db/migrate. Keep in mind that post migrations are executed post-deployment in production.
• Check timing guidelines for migrations
• Check migrations are reversible and implement a #down method
• Data migrations should be reversible too or come with a description of how to reverse, when possible. This applies to all types of migrations (regular, post-deploy, background).
• 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 or direct database access)
  • For given queries, review parameters regarding data distribution
  • Check query plans 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 in order to establish a proper testing environment.

### Timing guidelines for migrations

In general, migrations for a single deploy shouldn't take longer than 1 hour for GitLab.com. The following guidelines are not hard rules, they were estimated to keep migration timing to a minimum.

NOTE: Note: Keep in mind that all runtimes should be measured against GitLab.com.

Migration Type	Execution Time Recommended	Notes
Regular migrations on db/migrate	3 minutes	A valid exception are index creation as this can take a long time.
Post migrations on db/post_migrate	10 minutes
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.