Optimize QuerySet.values_list(flat=True) with no fields

[Adam Johnson]

Currently, QuerySet.values_list() ensures that no more than 1 field is set (​source):

        if flat and len(fields) > 1:
            raise TypeError(
                "'flat' is not valid when values_list is called with more than one "
                "field."
            )

However, it also allows the case where *no* fields are declared, for which all fields are fetched, only to throw away all but the first one (​source):

class FlatValuesListIterable(BaseIterable):
    """
    Iterable returned by QuerySet.values_list(flat=True) that yields single
    values.
    """

    def __iter__(self):
        queryset = self.queryset
        compiler = queryset.query.get_compiler(queryset.db)
        for row in compiler.results_iter(
            chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size
        ):
            yield row[0]

I think we can optimize this case to select only the first field in the model instead, maintaining semantics while avoiding overfetching.

This case also seems untested with the values_list() tests in tests/lookup/tests.py, so we'd want to add a test there.

[Simon Charette]

I wonder if we should take the opportunity to deprecate values_list(flat=True) instead by changing the check to be len(fields) != 1 as it seems like an omission in the API and should be a rare occurrence in code bases so I'm not convinced it's worth supporting it.