[fabianlindfors]

Your situation sounds just like what I have in mind for Reshape to fix! The backfills performed by Reshape are automatically done in batches to avoid locking.

As you said, Reshape can't call code outside the database as it relies on transactional guarantees for the migrations. Postgres has quite powerful support for procedural languages though so even though Reshape only supports SQL at the moment, there are opportunities to extend that in the future.

Reshape is currently Postgres only and uses a bunch of its features: schemas, updatable views, triggers, procedural functions, transactions and more. I'm sure equivalent things exist in MySQL for Reshape to work with that as well but I'm focusing on Postgres at the moment.