Add a deleting migrations guide

Include steps of how to disable a migration that was already executed on
GitLab.com on or self-hosted instances

doc/development/README.md

@@ -99,6 +99,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Post deployment migrations](post_deployment_migrations.md)
 - [Background migrations](background_migrations.md)
 - [Swapping tables](swapping_tables.md)
+- [Deleting exiting migrations](deleting_migrations.md)
 
 ### Best practices
 
@@ -118,7 +119,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Database helper modules](database_helpers.md)
 - [Code comments](code_comments.md)
 
-## Case studies
+### Case studies
 
 - [Database case study: Filtering by label](filtering_by_label.md)
 - [Database case study: Namespaces storage statistics](namespaces_storage_statistics.md)

doc/development/deleting_migrations.md

+# Delete existing migrations
+
+When removing existing migrations from the GitLab project, you have to take into account
+the possibility of the migration already been included in past releases or in the current release, and thus already executed on GitLab.com and/or in self-hosted instances.
+
+Because of it, it's not possible to delete existing migrations, as that could lead to:
+
+- Schema inconsistency, as changes introduced into the database were not rollbacked properly.
+- Leaving a record on the `schema_versions` table, that points out to migration that no longer exists on the codebase.
+
+Instead of deleting we can opt for disabling the migration.
+
+## Pre-requisites to disable a migration
+
+Migrations can be disabled if:
+
+- They caused a timeout or general issue on GitLab.com.
+- They are obsoleted, e.g. changes are not necessary due to a feature change.
+- Migration is a data migration only, i.e. the migration does not change the database schema.
+
+## How to disable a data migration?
+
+In order to disable a migration, the following steps apply to all types of migrations:
+
+1. Turn the migration into a noop by removing the code inside `#up`, `#down`
+  or `#perform` methods, and adding `#no-op` comment instead.
+1. Add a comment explaining why the code is gone.
+
+Disabling migrations requires explicit approval of Database Maintainer.
+
+## Examples
+
+- [Disable scheduling of productivity analytics](https://gitlab.com/gitlab-org/gitlab/merge_requests/17253)