Feature Request: schemadiff & OnlineDDL: support valid foreign key table cycles

[shlomi-noach]

Feature Description

VTGate's query serving distinguishes between valid and invalid foreign key loop references. For example, this pseudo code FK reference is valid:

users.avatar_id -> avatars.id
avatars.creator_id -> users.id

whereas the following is invalid:

t1.id -> t2.i
t2.i -> t1.id

In both cases we have two tables referencing each other. But in the first scenario, the referenced columns do not make a loop, where in the second scenario they do.

Right now schemadiff rejects table cycles altogether, just based on tables referencing each other. We want to make schemadiff compliant with query-serving logic and support valid cycles.

The output of schemadiff's diff should be then applicable to Online DDL.

We will create two distinct PRs:

• One, to support valid cycles in schemadiff.
• Another, to handle Online DDL.

Use Case(s)

Support valid foreign key schemas.

[shlomi-noach]

Closed by the combination of:

• schemadiff: support valid foreign key cycles #15431
• DDL strategy flag --unsafe-allow-foreign-keys implies setting FOREIGN_KEY_CHECKS=0 #15432
• MoveTables Atomic Mode: set FK checks off while deploying schema on targets #15448