In 5543677ec9 we introduced an optimization that uses Limit instead of
Unique to implement DISTINCT when all the DISTINCT pathkeys have been
marked as redundant. I happened to notice that this optimization was
not applied to partial DISTINCT, which I think should be. This can
improve plans in some cases, such as
-- on master
explain (costs off) select distinct four from tenk1 where four = 4;
QUERY PLAN
----------------------------------------------
Limit
-> Gather
Workers Planned: 4
-> Unique
-> Parallel Seq Scan on tenk1
Filter: (four = 4)
(6 rows)
-- patched
explain (costs off) select distinct four from tenk1 where four = 4;
QUERY PLAN
----------------------------------------------
Limit
-> Gather
Workers Planned: 4
-> Limit
-> Parallel Seq Scan on tenk1
Filter: (four = 4)
(6 rows)
Such queries might not be that common, but it's very cheap to apply this
optimization.
Attached is a patch for that.
Thanks
Richard
