Feature Request: introduce mysql DDL strategy, managed, but executed with MySQL DDL

[shlomi-noach]

Feature Description

We wish to introduce a new DDL strategy for Online DDL, termed mysql. A migration submitted with this strategy is managed, which means it is handled by the Online DDL scheduler. It goes through queued, ready, running, complete` phases. It can be retried or cancelled.

But the execution itself of the migration, specifically for ALTER TABLE, it to run the command directly on MySQL. This could happen to be a non-blocking migration (e.g. ALTER TABLE t ADD COLUMN c INT DEFAULT 0, ALGORITHM=INSTANT) or it could be a fully blocking operation. It's up to the specific query and the specific MySQL version running that query, as per https://dev.mysql.com/doc/refman/5.7/en/innodb-online-ddl-operations.html or https://dev.mysql.com/doc/refman/8.0/en/innodb-online-ddl-operations.html.

Notes:

• with this strategy it is impossible to CANCEL a running migration.
• there is no progress indication to a migration.
• ALTER migrations are non-revertible
• ALTER migrations do not support --postpone-completion
• ALTER migrations do not support --allow-concurrent

Use Case(s)

Some users will want to use MySQL as the engine because of:

• Limitations to the vitess/gh-ost strategies, such as specific key requirements
• To enforce running a migration as INSTANT or none at all (a migration submitted with ALTER TABLE ... ALGORITHM=INSTANT will fail if MySQL finds it impossible to apply the INSTANT algorithm)
• As a failsafe mechanism

But those users will enjoy the managed aspect of the migration (as opposed to direct):

• There is a record of the migration
• It can be reviewed
• It can be retried
• The scheduler will resolve concurrency issues, will queue and run the migration in the appropriate time

All in all, it's yet another option we give the user to choose from.

[shlomi-noach]

Closed by #12027