sql: support regr_avgx()

[obahareth]

Support aggregate function regr_avgx(). It referred to the PostgreSQL implementation, but the implementation details follow SQL 2003. See: postgresql.org/docs/10/functions-aggregate.html

See #41274

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This change is 

[obahareth]

I'm fixing the conflicts and pushing a better version of the tests.

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[yuzefovich]

Hey @obahareth, thanks for pushing this PR out! Is this ready for review?

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[obahareth]

Hey @yuzefovich, I should have fixed the failing tests/lint issues with this recent push. I think we should be good to go.

How should I handle the conflict with pkg/sql/execinfrapb/processors_sql.pb.go seeing as it's a generated file?

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[yuzefovich]

You need to regenerate those files and include the changes to your commit (I usually do make buildshort, but I think we also have make protobuf).

nit: we try to keep the commit messages with 80 character limit per line.
nit: the commit message should also contain a release note, something like

Release note (sql change): Aggregate function `regr_avgx` is now supported.

The code looks good to me with minor nits. We also should add some unit tests in aggregate_builtins_test.go similar to what we do for other statistics aggregates.

Reviewed 10 of 10 files at r1.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @obahareth)

---

pkg/sql/distsql/columnar_operators_test.go, line 75 at r1 (raw file):

	execinfrapb.AggregatorSpec_COVAR_POP:            2,
	execinfrapb.AggregatorSpec_COVAR_SAMP:           2,
	execinfrapb.AggregatorSpec_REGR_AVGX:            2,

nit: add it as the last entry so that the order is the same as in processors_sql.proto.

---

pkg/sql/sem/builtins/aggregate_builtins.go, line 2021 at r1 (raw file):

}

// regressionAvgXAggregate represents SQL:2003 average of the independent variable (sum(X)/N)

nit: we try to keep comments also at 80 character limit, and a period is missing.

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[obahareth]

There we go @yuzefovich, I believe I took care of all the comments you had, please let me know if I missed anything.

[yuzefovich]

Thanks for the contribution!

Note: the CI is red due to a flake.

bors r=yuzefovich

Reviewed 5 of 5 files at r2.
Reviewable status: complete! 1 of 0 LGTMs obtained

[craig]

Build succeeded:

• GitHub CI (Cockroach)

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