builtin: add cardinality builtin function

[jlevesy]

To ensure PostgreSQL compatibility, this commit adds a new builtin
function called cardinality which returns the total number of elements
present in a given array.

Fixes: #67996

Release note (sql change): Added cardinality builtin function that
returns the total number of elements in a given array.

[cockroach-teamcity]

All committers have signed the CLA.

[blathers-crl]

Thank you for contributing to CockroachDB. Please ensure you have followed the guidelines for creating a PR.

My owl senses detect your PR is good for review. Please keep an eye out for any test failures in CI.

I have added a few people who may be able to assist in reviewing:

• @rafiss (author of sql: implement CARDINALITY builtin for arrays #67996)

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[cockroach-teamcity]

This change is 

[rafiss]

thanks for your contribution! it's not exactly equivalent to array_length, just similar. the important difference is the way it handles empty arrays -- if you are able to try it out on postgres you'll see that cardinality returns 0 while array_length returns null.

please address that and add a test case

[blathers-crl]

Thank you for updating your pull request.

My owl senses detect your PR is good for review. Please keep an eye out for any test failures in CI.

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[rafiss]

this is getting close!

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @jlevesy)

---

pkg/sql/logictest/testdata/logic_test/builtin_function, line 1787 at r1 (raw file):

SELECT cardinality(ARRAY[]:::int[])
----
0

can you add one more test case for SELECT cardinality(NULL::int[]) -- it should return NULL

---

pkg/sql/sem/builtins/builtins.go, line 3134 at r1 (raw file):

			Fn: func(_ *tree.EvalContext, args tree.Datums) (tree.Datum, error) {
				arr := tree.MustBeDArray(args[0])
				return tree.NewDInt(tree.DInt(arr.Len())), nil

sorry to be nitpicky, but i'm worried that if we ever do support multi-dimensional arrays, this function will not work but we won't realize. can you add a recursive implementation? (here's what i'm thinking, borrowing from the arrayLength code)

func cardinality(arr *tree.DArray) tree.Datum {
	if arr.ParamTyp.Family() != types.ArrayFamily {
		return tree.NewDInt(tree.DInt(arr.Len()))
	}
	card := 0
	for _, a := range arr.Array {
		card += int(tree.MustBeDInt(cardinality(tree.MustBeDArray(a))))
	}
	return tree.NewDInt(tree.DInt(card))
}

[jlevesy]

Will do! Thanks for your reviews anyway 😄

[rafiss]

thanks for your contribution!

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)