sql: remove round-trips from common DISCARD ALL

[ajwerner]

sql: avoid checking if a role exists when setting to the current role

This is an uncached round-trip. It makes DISCARD expensive.
In #86485 we implemented
SET SESSION AUTHORIZATION DEFAULT, this added an extra sql query to
DISCARD ALL to check if the role we'd become exists. This is almost
certainly unintentional in the case where the role we'd become is the
current session role. I think this just happened because of code
consolidation.

We now no longer check if the current session role exists. This removes
the last round-trip from DISCARD ALL.

sql: use in-memory session data to decide what to do in DISCARD

In #86246 we introduced logic to discard schemas when running DISCARD ALL and DISCARD TEMP. This logic did expensive kv operations unconditionally; if the session knew it had never created a temporary schema, we'd still fetch all databases and proceed to search all databases for a temp schema. This is very expensive.

Fixes: #95864

Release note (performance improvement): In 22.2, logic was added to make
SET SESSION AUTHORIZATION DEFAULT not a no-op. This implementation used
more general code for setting the role for a session which made sure that
the role exists. The check for whether a role exists is currently uncached.
We don't need to check if the role we already are exists. This improves the
performance of DISCARD ALL in addition to SET SESSION AUTHORIZATION DEFAULT.

Release note (performance improvement): In 22.2, we introduced support for DISCARD TEMP and made DISCARD ALL actually discard temp tables. This implementation ran expensive logic to discover temp schemas rather than consulting in-memory data structures. As a result, DISCARD ALL, which is issued regularly by connection pools, became an expensive operation when it should be cheap. This problem is now resolved.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[ZhouXing19]

Thanks for the fix!

[ajwerner]

TFTR!

bors r+

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 66e265e to blathers/backport-release-22.2-95876: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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