Document the migration testing pipeline

doc/development/database/database_migration_pipeline.md

+---
+stage: Enablement
+group: Database
+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/#assignments
+---
+
+# Database migration pipeline
+
+> [Introduced](https://gitlab.com/gitlab-org/database-team/team-tasks/-/issues/171) in GitLab 14.2.
+
+With the [automated migration testing pipeline](https://gitlab.com/gitlab-org/database-team/gitlab-com-database-testing)
+we can automatically test migrations in a production-like environment (similar to `#database-lab`).
+It is based on an [architecture blueprint](../../architecture/blueprints/database_testing/index.md).
+
+Migration testing is enabled in the [GitLab project](https://gitlab.com/gitlab-org/gitlab)
+for changes that add a new database migration. Trigger this job manually by running the
+`db:gitlabcom-database-testing` job within in `test` stage. To avoid wasting resources,
+only run this job when your MR is ready for review. 
+
+The job starts a pipeline on the [ops GitLab instance](https://ops.gitlab.net/).
+For security reasons, access to the pipeline is restricted to database maintainers.
+
+When the pipeline starts, a bot notifies you with a comment in the merge request.
+When it finishes, the comment gets updated with the test results. 
+There are three sections which are described below.
+
+## Summary
+
+The first section of the comment contains a summary of the test results, including:
+
+| Result            | Description                                                                                                         |
+|-------------------|---------------------------------------------------------------------------------------------------------------------|
+| Warnings          | Highlights critical issues such as exceptions or long-running queries.                                              |
+| Migrations        | The time each migration took to complete, whether it was successful, and the increment in the size of the database. |
+| Runtime histogram | Expand this section to see a histogram of query runtimes across all migrations.                                     |
+
+## Migration details
+
+The next section of the comment contains detailed information for each migration, including: 
+
+| Result            | Description                                                                                                             |
+|-------------------|-------------------------------------------------------------------------------------------------------------------------|
+| Details           | The type of migration, total duration, and database size change.                                                        |
+| Queries           | Every query executed during the migration, along with the number of calls, timings, and the number of the changed rows. |
+| Runtime histogram | Indicates the distribution of query times for the migration.                                                            |
+
+## Clone details and artifacts
+
+Some additional information is included at the bottom of the comment:
+
+| Result                           | Description                                                                                                                                                                                                                                                     |
+|----------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| Migrations pending on GitLab.com | A summary of migrations not deployed yet to GitLab.com. This info is useful when testing a migration that was merged but not deployed yet.                                                                                                                      |
+| Clone details                    | A link to the `Postgres.ai` thin clone created for this testing pipeline, along with information about its expiry. This can be used to further explore the results of running the migration. Only accessible by database maintainers or with an access request. |
+| Artifacts                        | A link to the pipeline's artifacts. Full query logs for each migration (ending in `.log`) are available there and only accessible by database maintainers or with an access request.                                                                            |

doc/development/database_review.md

@@ -117,7 +117,7 @@ test its execution using `CREATE INDEX CONCURRENTLY` in the `#database-lab` Slac
   - If the execution from `#database-lab` is longer than `1h`, the index should be moved to a [post-migration](post_deployment_migrations.md).
     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
     will be in place when the code that needs it will be deployed.
-- Trigger the [database testing](../architecture/blueprints/database_testing/index.md) job (`db:gitlabcom-database-testing`) in the `test` stage.
+- Manually trigger the [database testing](database/database_migration_pipeline.md) job (`db:gitlabcom-database-testing`) in the `test` stage.
   - This job runs migrations in a production-like environment (similar to `#database_lab`) and posts to the MR its findings (queries, runtime, size change).
   - Review migration runtimes and any warnings.