sql: be safer about swapping out IVarHelpers

[justinj]

Fixes #22249.

This fix is admittedly somewhat ad-hoc, though I argue that it is
strictly less ad-hoc than the system we had before! The provided logic
test would panic because there was a place in the code where we were not
appropriately resetting an IVarHelper after swapping it out, leaving an
EvalContext with a nil IVarHelper.

I think this swapping out of IVarHelpers is something that has evolved
organically and has never been a particularly principled thing for us to
do. We should at some point figure out what it means for us to evaluate
a new expression with different indexed var bindings with the same
"context".

Release note (bug fix): Fix a panic with certain queries involving
REGCLASS

[cockroach-teamcity]

This change is 

[knz]

Looks good!

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Reviewed 8 of 8 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks pending.

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pkg/sql/join_predicate.go, line 321 at r1 (raw file):

		copy(p.curRow[:len(leftRow)], leftRow)
		copy(p.curRow[len(leftRow):], rightRow)
		oldIVarHelper := ctx.IVarHelper

curious: why no push/pop here?

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Comments from Reviewable

[justinj]

Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks pending.

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pkg/sql/join_predicate.go, line 321 at r1 (raw file):

Previously, knz (kena) wrote…

curious: why no push/pop here?

Oversight! I grepped for IVarHelper = nil. Will fix.

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Comments from Reviewable

[justinj]

Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks pending.

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pkg/sql/join_predicate.go, line 321 at r1 (raw file):

Previously, justinj (Justin Jaffray) wrote…

Oversight! I grepped for IVarHelper = nil. Will fix.

Oh I see I added that line, it was still an oversight, this was me experimenting and I forgot to change it back.

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Comments from Reviewable

[knz]

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Reviewed 2 of 2 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[justinj]

TFTR!