It might be better to return a string column with multiple rows, one row for each index recommendation, than an array of strings? What do you think? You can look at set-returning built-ins for inspiration: https://github.com/cockroachdb/cockroach/blob/42634711a5869bad8cde5677176cdec7a8f507d0/pkg/sql/sem/builtins/generator_builtins.go

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pkg/sql/sem/builtins/generator_builtins_test.go line 86 at r2 (raw file):

		require.Equalf(t, false, rows.Next(), "failed test case %d", i+1)
	}
}

It'd be more consistent with the rest of the tests if these were logic tests. Is there a reason not to put them in a logic test like pkg/sql/logictest/testdata/logic_test/workload_recs?

sorry you've been waiting so long for a review! Just a few nits below.

Reviewed 1 of 5 files at r2, 11 of 11 files at r3, all commit messages.
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-- commits line 24 at r3:
I think this should have a release note, since these functions will be user-facing

pkg/sql/sem/builtins/generator_builtins.go line 285 at r3 (raw file):

			tree.ParamTypes{},
			types.String,
			makeWorkloadIndexRecsGeneratorFactory(false, false),

nit: for each of these boolean parameters, we usually add a comment to say what it is. Something like this (here and below):

makeWorkloadIndexRecsGeneratorFactory(false /* hasTs */, false /* hasBgt */),

pkg/sql/sem/builtins/generator_builtins.go line 1121 at r3 (raw file):

// makeWorkloadIndexRecsGeneratorFactory uses the arrayValueGenerator to
// return all the index recommendations as an array of strings
func makeWorkloadIndexRecsGeneratorFactory(hasTs bool, hasBgt bool) eval.GeneratorOverload {

nit: I'd spell these params out: hasTimestamp and hasBudget. Also update the function comment to mention what these params mean.

pkg/sql/sem/builtins/generator_builtins.go line 1123 at r3 (raw file):

func makeWorkloadIndexRecsGeneratorFactory(hasTs bool, hasBgt bool) eval.GeneratorOverload {
	return func(_ context.Context, _ *eval.Context, _ tree.Datums) (eval.ValueGenerator, error) {
		// Invoke the workloadindexrec.FindWorkloadRecs() to get indexRecs, err once it is implemented.

nit: comment should be <= 80 characters wide. you can just wrap the text here.

pkg/sql/logictest/testdata/logic_test/workload_indexrecs line 1 at r3 (raw file):

# get workload index-recs

It would be good to add a comment to the top of this file mentioning that the data returned right now is just dummy data.

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-- commits line 24 at r3:

Fine with me, as long as we don't forget to add one eventually (I think the docs team usually does some massaging anyway, so it's also fine to add something in both places)