Adapt sumMap to accept an array of strings as the key column. This is useful when we do not want or cannot make the keys be numbers.
Signed-off-by: Baudouin Giard <bgiard@bloomberg.net>
The function is rewritten to avoid allocations on every insert with
Field deserialising each array. The key type is now specialized,
so it can be accessed directly. The value type is variant type,
but only individual values are deserialised (which is cheap, since they're PODs).
The function also support summing of multiple columns by the same key.
The SummingSortedBlockInputStream uses the function in case of
Nested structure with one numeric key and many values to sum.
