Add union_tag scalar function

[gstvg]

Which issue does this PR close?

• Closes Add union_tag function #11080

Rationale for this change

Retrieve the name of the currently selected field on a union, as there's no way to do it today

What changes are included in this PR?

union_tag scalar function implementation

Are these changes tested?

Yes, with sqllogictests when possible, and with unit tests for union scalars, which are not supported in SQL yet

Are there any user-facing changes?

A new scalar function union_tag

[gstvg]

The union column used on the sqllogictests contains a single field with type id 3, so this is put to the test

datafusion/datafusion/sqllogictest/src/test_context.rs

Lines 411 to 430 in e4b78c7

	fn register_union_table(ctx: &SessionContext) {
	let union = UnionArray::try_new(
	UnionFields::new(vec![3], vec![Field::new("int", DataType::Int32, false)]),
	ScalarBuffer::from(vec![3, 3]),
	None,
	vec![Arc::new(Int32Array::from(vec![1, 2]))],
	)
	.unwrap();
	let schema = Schema::new(vec![Field::new(
	"union_column",
	union.data_type().clone(),
	false,
	)]);
	let batch =
	RecordBatch::try_new(Arc::new(schema.clone()), vec![Arc::new(union)]).unwrap();
	ctx.register_batch("union_table", batch).unwrap();
	}

datafusion/datafusion/sqllogictest/src/test_context.rs

Lines 117 to 120 in e4b78c7

	"union_function.slt" => {
	info!("Registering table with union column");
	register_union_table(test_ctx.session_ctx())
	}

[Omega359]

@alamb - here is another function coming in (xxhash, regexp_extract (both versions of it), array_min/array_max functions) where it is not clear what should be accepted and what shouldn't be. Since we've already accepted union_extract this may be a case of fleshing that series of functions out.

I strongly think we need 'official' documentation as to what will be and won't be accepted and an recommended repository/sub project where additional functions can be located. Hopefully under the apache umbrella such that they hopefully can maintained by the community and work across multiple DF versions.

[alamb]

@alamb - here is another function coming in (xxhash, regexp_extract (both versions of it), array_min/array_max functions) where it is not clear what should be accepted and what shouldn't be. Since we've already accepted union_extract this may be a case of fleshing that series of functions out.

I strongly think we need 'official' documentation as to what will be and won't be accepted and an recommended repository/sub project where additional functions can be located. Hopefully under the apache umbrella such that they hopefully can maintained by the community and work across multiple DF versions.

Yeah I agree. I think we should file a "discussion" type ticket to have this discussion. I can file one at some point later (I am low on time this week) or if you can that would be sweet.

we have some generic guidance here: https://datafusion.apache.org/contributor-guide/index.html#what-contributions-are-good-fits

[Omega359]

Yeah I agree. I think we should file a "discussion" type ticket to have this discussion. I can file one at some point later (I am low on time this week) or if you can that would be sweet.

we have some generic guidance here: https://datafusion.apache.org/contributor-guide/index.html#what-contributions-are-good-fits

on it. #14777