VReplication: Properly Handle FK Constraints When Deferring Secondary Keys

[mattlord]

Description

The MoveTables and Reshard commands/workflows support deferring secondary key generation until after the copy phase completes — adding all of them in one shot per table — via the --defer-secondary-keys flag.

In v18 we began adding full support for foreign keys and in the process of doing that work we realized that when deferring secondary keys the workflow would fail with an error that a secondary key could not be dropped because it was needed for a foreign key. For example, the new unit test cases fail this way on main:

❯ go test -timeout 30s -run ^TestDeferSecondaryKeys$ vitess.io/vitess/go/vt/vttablet/tabletmanager/vreplication
--- FAIL: TestDeferSecondaryKeys (0.16s)
    --- FAIL: TestDeferSecondaryKeys/2SK:1FK (0.01s)
        vreplicator_test.go:474:
            	Error Trace:	/Users/matt/git/vitess/go/vt/vttablet/tabletmanager/vreplication/vreplicator_test.go:474
            	Error:      	Received unexpected error:
            	            	Cannot drop index 't2_id': needed in a foreign key constraint (errno 1553) (sqlstate HY000) during query: alter table vrepl.t1 drop key c1, drop key c2, drop key t2_id
            	Test:       	TestDeferSecondaryKeys/2SK:1FK

Even if this did work, we should not remove those keys so that we can utilize the foreign keys as expected during the workflow's copy phase in the future.

In this PR we add a check to see if the secondary key is required for a foreign key constraint and if so we do not temporarily remove it during the copy phase.

Related Issue(s)

• Fixes: Bug Report: Deferring Secondary Keys Does Not Work With Foreign Keys #14547

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

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[deepthi]

Should this really be back ported to v18? I thought we were not back porting anything FK-related because it is experimental.

[mattlord]

Should this really be back ported to v18? I thought we were not back porting anything FK-related because it is experimental.

I thought it was worth doing, but we don't have to.