Allow DISTINCT aggregates pushdown if partition keys are equal

CREATE INDEX ix1 ON default(c1,c2,c3,c4) PARTITION BY HASH(c1,c3);

SELECT COUNT(DISTINCT d.c2)

FROM default AS d

WHERE d.c1 = 5 AND d.c3 = 7 AND d.c2 IS NOT NULL and d.c4 BETWEEN 10 AND 20;

In above case HASH on c1,c3 AND equality predicate i.e. it will resolve single partition (All partition keys must be equality predicates), So treat this same as non partition index and apply same rules.