feat: add support for emit in aggregate relations

[EpsilonPrime]

This adds support for emit in aggregations where there is no more than one grouping section.

This addresses #121 .

[bkietz]

For pattern matching like this, I recommend adding a conditional cast macro:

#define ANY_CAST_IF(ValueType, value) \
  value.type() != typeid(ValueType) \
  ? ::std::nullopt \
  : ::std::make_optional(ANY_CAST(ValueType, value))

Then you can write

Suggested change

	if (symbol->subtype.type() == typeid(::substrait::proto::Rel::RelTypeCase) &&
	ANY_CAST(::substrait::proto::Rel::RelTypeCase, symbol->subtype) ==
	::substrait::proto::Rel::kAggregate) {
	return true;
	}
	if (auto type_case = ANY_CAST_IF(::substrait::proto::Rel::RelTypeCase, symbol->subtype)) {
	return type_case == ::substrait::proto::Rel::kAggregate;
	}

If you want to be even fancier, I can provide a match() helper:

bool isAggregate(const SymbolInfo* symbol) {
  return match(symbol->subtype,
    [](::substrait::proto::Rel::RelTypeCase type_case) {
      return type_case == ::substrait::proto::Rel::kAggregate;
    },
    [](RelationType rel_type) {
      return rel_type == RelationType::kAggregate;
    },
    [](...) {
      return false;
    });
}

[bkietz]

This seems odd to me because I'd expect a clean outer branch on the type of currentScope before we look at the value of fieldReference. If currentScope is an aggregate but fieldReference is out of range, is it really reasonable to use the same code we would have used if currentScope were not an aggregate? I'd expect something like:

Suggested change

	if (isAggregate(currentScope) &&
	fieldReference < relationData->generatedFieldReferences.size()) {
	symbol = relationData->generatedFieldReferences[fieldReference];
	} else if (fieldReference < fieldReferencesSize) {
	symbol = relationData->fieldReferences[fieldReference];
	} else if (
	fieldReference <
	fieldReferencesSize + relationData->generatedFieldReferences.size()) {
	symbol =
	relationData
	->generatedFieldReferences[fieldReference - fieldReferencesSize];
	} else {
	errorListener_->addError(
	"Encountered field reference out of range: " +
	std::to_string(fieldReference));
	return "field#" + std::to_string(fieldReference);
	}
	if (isAggregate(currentScope)) {
	if (fieldReference < relationData->generatedFieldReferences.size()) {
	symbol = relationData->generatedFieldReferences[fieldReference];
	}
	} else {
	auto size = relationData->fieldReferences.size();
	if (fieldReference < size) {
	symbol = relationData->fieldReferences[fieldReference];
	} else if (
	fieldReference <
	size + relationData->generatedFieldReferences.size()) {
	symbol =
	relationData
	->generatedFieldReferences[fieldReference - size];
	}
	}
	if (symbol == nullptr) {
	errorListener_->addError(
	"Encountered field reference out of range: " +
	std::to_string(fieldReference));
	return "field#" + std::to_string(fieldReference);
	}

in any case, please clarify the branching here

[bkietz]

Suggested change

	" beyond what the aggregate produced.");
	") beyond what the aggregate produced.");

[bkietz]

Suggested change

	return symbol->subtype.type() == typeid(RelationType) &&
	ANY_CAST(RelationType, symbol->subtype) == RelationType::kAggregate;
	return ANY_CAST_IF(RelationType, symbol->subtype) == RelationType::kAggregate;