release-22.1: colbuilder: fix aggregation with no aggregate funcs

[blathers-crl]

Backport 1/1 commits from #87662 on behalf of @yuzefovich.

/cc @cockroachdb/release

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Previously, if there are no aggregate functions to compute, we would plan a special "num fixed tuples" operator that would always return a single tuple when in scalar and no tuples when in non-scalar context, but this is incorrect - we should be returning the same number of tuples as there are groups for non-empty input. The previous setup was correct only when the input is empty. This is now fixed by teaching the ordered aggregator to handle this special case instead. The impact of the bug seems minor since the optimizer should only be creating such plans when some of its rules are disabled.

Fixes: #87619.

Release note: None (this shouldn't be a user-visible bug)

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Release justification: bug fix.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[yuzefovich]

Removed the regression test since the settings to disable rules aren't available on 22.1.

[DrewKimball]

I should have asked this in the original PR, but does the row engine correctly handle this case? I don't think it will ever be used if vectorize=on, but should be if it's off. We should probably also add a vectorize=off version of the test case.

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @msirek and @yuzefovich)

[msirek]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @yuzefovich)

[yuzefovich]

does the row engine correctly handle this case?

Yeah, it does - this is actually how we found the bug (due to the difference between outputs of two engines). Already included regression test to vectorize_agg does run with local-vec-off and fakedist-vec-off, so we're covered there (on 22.1 we don't have a repro, so there is no regression test on this PR).