release-22.1: sql/stats: support rowCountEq = 0 in histogram.adjustCounts

[michae2]

Backport 1/1 commits from #82474.

/cc @cockroachdb/release

---

The predicted histograms in statistics forecasts will often have buckets
with NumEq = 0, and some predicted histograms will have all buckets
with NumEq = 0. This wasn't possible before forecasting, because the
histograms produced by EquiDepthHistogram never have any buckets with
NumEq = 0.

If adjustCounts is called on such a histogram, rowCountEq and
distinctCountEq will be zero. adjustCounts should still be able to
fix such a histogram to have sum(NumRange) = rowCountTotal and
sum(DistinctRange) = distinctCountTotal. This patch teaches
adjustCounts to handle these histograms.

(Similarly, predicted histograms could have all buckets with
NumRange = 0, but this is already possible for histograms produced by
EquiDepthHistogram, so adjustCounts already handles these.)

Also, add a few more comments to adjustCounts.

Assists: #79872

Release note: None

---

Release justification: Low risk, high benefit change to existing functionality.

[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 

[msirek]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner and @rytaft)

[michae2]

Thanks! I'll let this bake a little longer on master before merging.

[rytaft]

I know this will be needed for ultimately backporting the solution to #79872, but I'm starting to question whether backporting all the changes that will be needed is wise. Let's hold off on merging this until it's clear that we really do want to backport the comprehensive fix to #79872.

[michae2]

Closing this, will reopen later if we do decide to backport.