209 lines
12 KiB
Markdown
209 lines
12 KiB
Markdown
---
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stage: Enablement
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group: Database
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info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
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---
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# 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. For example:
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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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- Changes in usage data metrics that use `count` and `distinct_count`.
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These metrics could have complex queries over large tables.
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See the [Product Analytics Guide](product_analytics/usage_ping.md#implementing-usage-ping)
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for implementation details.
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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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- [Prepare the merge request for a database review](#how-to-prepare-the-merge-request-for-a-database-review).
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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-foss/-/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 the merge request for a database review
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In order to make reviewing easier and therefore faster, please take
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the following preparations into account.
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#### Preparation when adding migrations
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- Ensure `db/structure.sql` is updated as [documented](migration_style_guide.md#schema-changes), and additionally ensure that the relevant version files under
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`db/schema_migrations` were added or removed.
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- Make migrations reversible by using the `change` method or include a `down` method when using `up`.
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- Include either a rollback procedure or describe how to rollback changes.
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- Add the output of both migrating and rolling back for all migrations into the MR description.
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- Ensure the down method reverts the changes in `db/structure.sql`.
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- Update the migration output whenever you modify the migrations during the review process.
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- Add tests for the migration in `spec/migrations` if necessary. See [Testing Rails migrations at GitLab](testing_guide/testing_migrations_guide.md) for more details.
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- When [high-traffic](https://gitlab.com/gitlab-org/gitlab/-/blob/master/rubocop/rubocop-migrations.yml#L3) tables are involved in the migration, use the [`with_lock_retries`](migration_style_guide.md#retry-mechanism-when-acquiring-database-locks) helper method. Review the relevant [examples in our documentation](migration_style_guide.md#examples) for use cases and solutions.
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- Ensure RuboCop checks are not disabled unless there's a valid reason to.
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- When adding an index to a [large table](https://gitlab.com/gitlab-org/gitlab/-/blob/master/rubocop/rubocop-migrations.yml#L3),
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test its execution using `CREATE INDEX CONCURRENTLY` in the `#database-lab` Slack channel and add the execution time to the MR description:
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- Execution time largely varies between `#database-lab` and GitLab.com, but an elevated execution time from `#database-lab`
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can give a hint that the execution on GitLab.com will also be considerably high.
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- If the execution from `#database-lab` is longer than `1h`, the index should be moved to a [post-migration](post_deployment_migrations.md).
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Keep in mind that in this case you may need to split the migration and the application changes in separate releases to ensure the index
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will be in place when the code that needs it will be deployed.
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#### Preparation when adding or modifying queries
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- Write the raw SQL in the MR description. Preferably formatted
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nicely with [pgFormatter](https://sqlformat.darold.net) or
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[paste.depesz.com](https://paste.depesz.com).
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- Include the output of `EXPLAIN (ANALYZE, BUFFERS)` of the relevant
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queries in the description. If the output is too long, wrap it in
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`<details>` blocks, paste it in a GitLab Snippet, or provide the
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link to the plan at: [explain.depesz.com](https://explain.depesz.com).
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- When providing query plans, make sure it hits enough data:
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- You can use a GitLab production replica to test your queries on a large scale,
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through the `#database-lab` Slack channel or through [chatops](understanding_explain_plans.md#chatops).
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- Usually, the `gitlab-org` namespace (`namespace_id = 9970`) and the
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`gitlab-org/gitlab-foss` (`project_id = 13083`) or the `gitlab-org/gitlab` (`project_id = 278964`)
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projects provide enough data to serve as a good example.
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- For query changes, it is best to provide the SQL query along with a
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plan _before_ and _after_ the change. This helps to spot differences
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quickly.
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- Include data that shows the performance improvement, preferably in
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the form of a benchmark.
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#### Preparation when adding foreign keys to existing tables
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- Include a migration to remove orphaned rows in the source table **before** adding the foreign key.
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- Remove any instances of `dependent: ...` that may no longer be necessary.
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#### Preparation when adding tables
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- Order columns based on the [Ordering Table Columns](ordering_table_columns.md) guidelines.
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- Add foreign keys to any columns pointing to data in other tables, including [an index](migration_style_guide.md#adding-foreign-key-constraints).
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- Add indexes for fields that are used in statements such as `WHERE`, `ORDER BY`, `GROUP BY`, and `JOIN`s.
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#### Preparation when removing columns, tables, indexes, or other structures
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- Follow the [guidelines on dropping columns](what_requires_downtime.md#dropping-columns).
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- Generally it's best practice (but not a hard rule) to remove indexes and foreign keys in a post-deployment migration.
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- Exceptions include removing indexes and foreign keys for small tables.
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- If you're adding a composite index, another index might become redundant, so remove that in the same migration.
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For example adding `index(column_A, column_B, column_C)` makes the indexes `index(column_A, column_B)` and `index(column_A)` redundant.
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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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- If an index to a large table is added and its execution time was elevated (more than 1h) on `#database-lab`:
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- Ensure it was added in a post-migration.
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- Maintainer: After the merge request is merged, notify Release Managers about it on `#f_upcoming_release` Slack channel.
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- Check consistency with `db/structure.sql` and that migrations are [reversible](migration_style_guide.md#reversibility)
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- Check that the relevant version files under `db/schema_migrations` were added or removed.
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- Check queries timing (If any): Queries executed in a migration
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need to fit comfortably within `15s` - preferably much less than that - on GitLab.com.
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- For column removals, make sure the column has been [ignored in a previous release](what_requires_downtime.md#dropping-columns)
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- Check [background migrations](background_migrations.md):
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- Establish a time estimate for execution on GitLab.com. For historical purposes,
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it's highly recommended to include this estimation on the merge request description.
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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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- Background migrations are normally used, but not limited to:
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- Migrating data in larger tables.
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- Making numerous SQL queries per record in a dataset.
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- Review queries (for example, make sure batch sizes are fine)
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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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- Check migrations are reversible and implement a `#down` method
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- Check data migrations:
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- Establish a time estimate for execution on GitLab.com.
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- Depending on timing, data migrations can be placed on regular, post-deploy, or background migrations.
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- Data migrations should be reversible too or come with a description of how to reverse, when possible.
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This applies to all types of migrations (regular, post-deploy, background).
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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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(for example, 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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- Avoid N+1 problems and minimalize the [query count](merge_request_performance_guidelines.md#query-counts).
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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:**
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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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| 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 single query must stay below `1 second` execution time with cold caches. |
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