Commit 3d706635 authored by Achilleas Pipinellis's avatar Achilleas Pipinellis

Merge branch 'docs-on-removing-fk-and-locking' into 'master'

Add migration docs about removing a foreign key

See merge request gitlab-org/gitlab!77204
parents 624e39dc 4946aa8c
......@@ -772,6 +772,32 @@ to run on a large table, as long as it is only updating a small subset of the
rows in the table, but do not ignore that without validating on the GitLab.com
staging environment - or asking someone else to do so for you - beforehand.
## Removing a foreign key constraint
When removing a foreign key constraint, we need to acquire a lock on both tables
that are related to the foreign key. For tables with heavy write patterns, it's a good
idea to use `with_lock_retries`, otherwise you might fail to acquire a lock in time.
You might also run into deadlocks when acquiring a lock, because ordinarily
the application writes in `parent,child` order. However, removing a foreign
key acquires the lock in `child,parent` order. To resolve this, you can
explicitly acquire the lock in `parent,child`, for example:
```ruby
disable_ddl_transaction!
def up
with_lock_retries do
execute('lock table ci_pipelines, ci_builds in access exclusive mode')
remove_foreign_key :ci_builds, to_table: :ci_pipelines, column: :pipeline_id, on_delete: :cascade, name: 'the_fk_name'
end
end
def down
add_concurrent_foreign_key :ci_builds, :ci_pipelines, column: :pipeline_id, on_delete: :cascade, name: 'the_fk_name'
end
```
## Dropping a database table
Dropping a database table is uncommon, and the `drop_table` method
......
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