feat: Support Union type in approx_distinct

[mkleen]

Which issue does this PR close?

• Relates to Support more types for approx_distinct function #22989 but does not close it. More types are coming.

Rationale for this change

• Support the Arrow type Union for approx_distinct

What changes are included in this PR?

• Enable HLLAccumulator and HllGroupsAccumulator to support Union
• Tests for the non-grouped and grouped path as part of approx_distinct.rs

Are these changes tested?

Yes

Are there any user-facing changes?

Yes, approx_distinct supports now Union but no breaking changes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.69%. Comparing base (3b1ce16) to head (9096c52).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #23714   +/-   ##
=======================================
  Coverage   80.69%   80.69%           
=======================================
  Files        1088     1088           
  Lines      367766   367813   +47     
  Branches   367766   367813   +47     
=======================================
+ Hits       296781   296820   +39     
- Misses      53276    53279    +3     
- Partials    17709    17714    +5     

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[Jefffrey]

not possible to do via SLT?

[mkleen]

It's tricky via SLT. There is no SQL union type but only union operations in datafusion.

[Jefffrey]

what if we add more rows here:

datafusion/datafusion/sqllogictest/src/test_context.rs

Lines 560 to 594 in 3a2bc0a

	fn register_union_table(ctx: &SessionContext) {
	let union = UnionArray::try_new(
	UnionFields::try_new(
	// typeids: 3 for int, 1 for string
	vec![3, 1],
	vec![
	Field::new("int", DataType::Int32, false),
	Field::new("string", DataType::Utf8, false),
	],
	)
	.unwrap(),
	ScalarBuffer::from(vec![3, 1, 3]),
	None,
	vec![
	Arc::new(Int32Array::from(vec![1, 2, 3])),
	Arc::new(StringArray::from(vec![
	Some("foo"),
	Some("bar"),
	Some("baz"),
	])),
	],
	)
	.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();
	}

then can use in slts like here

datafusion/datafusion/sqllogictest/test_files/union_function.slt

Lines 18 to 30 in 3a2bc0a

	# Note: union_table is registered via Rust code in the sqllogictest test harness
	# because there is no way to create a union type in SQL today
	##########
	## UNION DataType Tests
	##########
	query ?I
	select union_column, union_extract(union_column, 'int') from union_table;
	----
	{int=1} 1
	{string=bar} NULL
	{int=3} 3