Feature Request: query buffering, transaction termination, for "special plan" migrations such as `INSTANT` DDL

[shlomi-noach]

Feature Description

vreplication based migrations enjoy a sophisticated cut-over mechanism, which among other things and based on configuration, supports:

• Query buffering for the duration of the cut-over phase (up to a limit).
• Forced termination of queries and transactions that are holding locks or metadatalocks on the table, just before cut-over.

We wish to extend the above two to "special plan" ALTER TABLE operations, which currently include:

• INSTANT DDL (and when --prefer-instant-ddl strategy flag is specified)
• Some partition rotation operations (create new range partition, drop old range partitions)

These "special plan" operations are technically just handed off to MySQL. Both buffering and forced termination of queries/transactions can help mitigate a scenario where the changes are stalled due to a high workload.

At this time the proposal is to issue a forced termination just once and immediate before running the migration. In the future we might look into terminating queries/transactions that are affecting the migration even while it is being stalled.

Use Case(s)

Schema changes under heavy workloads