no issue
For large numbers of members we're unable to perform queries for free/paid members in a reasonable time without indexes. Bulk delete is also very slow when looping through bookshelf models and having bookshelf manage deletion of child records, adding `ON DELETE CASCADE` to the foreign key indexes moves child deletion to the DB level and allows for performant bulk delete queries.
- migrations only run on mysql because sqlite does not support altering tables
- adds foreign key indexes and constraints with 'ON DELETE CASCADE' for members_labels, members_stripe_customers, and members_stripe_customers_subscriptions
refs #12100
For performance reasons we want to add foreign key and unique constraints
to the members_stripe_* tables so we can utilised cascading deletes and
joins across the tables when querying.
In order to do this we must first ensure that:
- There are no duplicate entries in the `subscription_id` or `customer_id` columns
- There are no orphaned rows in the subscription or customers tables
If the first is not true, the unique constraint will fail, and if the second is not true,
the foreign key constraint will fail.
As we are only adding the indexes to existing MySQL databases at this point, the
cleanup migrations will also only be done for existing MySQL databases too.
The migrations for removing orphaned rows splits the deletion into a `SELECT`
followed by a `WHERE IN` to avoid the database "optimising" the query into a
`JOIN` which ends up taking much longer due to the lack of indexes.
