Pushdown DISTINCT agg fields as (additional) GROUP BY keys

[kaikalur]

DISTINCT aggs like COUNT DISTINCT are expensive. But in most cases it is helpful to push down these as group by keys before counting them.

SELECT agg(x), COUNT(DISTINCT y) FROM T GROUP BY z;

can be rewritten as

SELECT agg(x), COUNT(DISTINCT y) FROM (SELECT partial_agg(x) AS x, y, z FROM T GROUP BY y, z) AS T GROUP BY z;