sql: boostrap a user-controlled o11y db for SQL obs

[knz]

See internal proposal.

Jira issue: CRDB-30793

Epic CRDB-31701

[knz]

@fqazi i'd like to pick your brain on the topic of "automatically re-routing queries based on migration stage".

As you can see in #109143, I was able to side-step the idea of "a hack in name resolution" by making the queries refer to the database implicitely. This gets us 90% of the way there.

There's just one step remaining: there's a couple of SQL views in crdb_internal (e.g. crdb_internal.statement_statistics) that also refer to system. explicitly.

How can I make these refer to the system.XXX table with a variable database prefix?

I can see a couple of different approaches:

• move the part of the view query that refers to system.XXX into a (new) virtual table which conditionally selects the database depending on context. I believe this is the easiest.
• change the view query to not hardcode the system. prefix, then change virtualSchemaView and all the related machinery to add a new boolean flag that will reroute the "current db" parent in the query planner.

Maybe you have other ideas?

[fqazi]

Both options are reasonable, but the second option won't be needed once the mixed version state is gone/removed for good. I can't think of any other choices since we would need to influence name resolution, and the scope has to be narrow.

[maryliag]

No longer a priority