Improve FK Performance and Compatibility

[jseldess]

Background: https://cockroachlabs.atlassian.net/browse/CRDB-779

Team: Andrew Woods, Radu Berinde

Github Tracking Issue: cockroachdb/cockroach#15157 cockroachdb/cockroach#25809 cockroachdb/cockroach#26786 cockroachdb/cockroach#26795 cockroachdb/cockroach#23580 https://github.com/cockroachlabs/support/issues/224 cockroachdb/cockroach#33467 cockroachdb/cockroach#16491 #4003 cockroachdb/cockroach#36859 #4788 #5967

[ericharmeling]

I'm going to bump this item to 20.2, as not all of these feature updates are going to be ready for 20.1.

We can document the individual improvements in FK performance as they come up in release notes.

[jseldess]

@ericharmeling, I'm sorry it's hard to track this one. Can you track down the relevant release note issues and confirm with @awoods187 and devs that they cover all of the improvements in 20.1?

[awoods187]

Hey @ericharmeling. We do have a bit of work to do here. Foreign Keys are now planned by the CBO and no longer require an experimental flag. What got bumped is Foreign Key Cascade which still use the old implementation. This is a small docs project to remove the experimental flag and mention that they are planned using the CBO. After FK cascade lands in 20.2, we will have another small project to update that FKs don't use an extra index but mostly the user will just experience better FK performance. cc @RaduBerinde