CopySchemaShard: apply changes with foreign key checks disabled

[dweitzman]

People who don't have foreign keys won't care, and people who
do have FKs need the checks disabled because the logic is not yet clever enough
to create the tables in an order which is compatible with their
foreign key dependencies

[dweitzman]

Not yet tested

[sougou]

Do you think it's better to do this at the beginning of SplitClone instead? And then, SplitClone can enable checks just before setting up vreplication.

I can accordingly change vreplication copy do the same thing.

[dweitzman]

CopySchemaShard will need to run before SplitClone, so it seems like they both need something like this.

[sougou]

CopySchemaShard itself should succeed without the check, right? The tables have no data in them.

[derekperkins]

It still errors out if the tables aren't created in the right order.

[sougou]

Ahh, of course...
Should re-enable checks after the apply?
Then we should have SplitClone disable for its own sake.

[dweitzman]

Ahh, of course...
Should re-enable checks after the apply?
Then we should have SplitClone disable for its own sake.

set foreign_key_checks = 0 should only be setting a session variable, not a global, so if usePool=false does what it sounds like it does I would think there shouldn't be a need to reset it on the connection after copying the schema.

I could potentially add set foreign_key_checks = @@foreign_key_checks afterwards to copy the global back into the session as a precaution in case someone removes the usePool=false in the future. Was trying to discourage that with a comment, but a little extra caution seems harmless. I don't have strong feelings either way.

What do you think?

[sougou]

In that case, this is good to go. I've restarted the failed tests. We can merge once they pass.

[sougou]

Looks like the tests need fixing after all.

Tests need fixing

[dweitzman]

Oh, I thought a dba query would allow running an arbitrary set but that must not be true. Need to look into this:

TabletManager.ExecuteFetchAsDba on cell1-0000000010 error: rpc error: code = Unknown desc = unknown error: query: 'set foreign_key_checks = 0' is not supported on destinationMasterDb (errno 1105) (sqlstate HY000) during query: set foreign_key_checks = 0: rpc error: code = Unknown desc = unknown error: query: 'set foreign_key_checks = 0' is not supported on destinationMasterDb (errno 1105) (sqlstate HY000) during query: set foreign_key_checks = 0

[sougou]

I think these tests use mocks. You'll just have to add that query to the expected list.

[sougou]

@dweitzman we should rework this PR into the newer vreplication. Disable FK checks during vrepl and enable it just before accepting writes in MigrateWrites.

[deepthi]

This has been addressed in #6284. Please feel free to re-open if that doesn't cover all situations.