I tried both of those and concluded they'd be too messy for a patch that we might find ourselves having to back-patch. So 0001 attached fixes it by teaching SS_finalize_plan to treat optimized MIN()/MAX() aggregates as if they were already Params. It's slightly annoying to have knowledge of that optimization metastasizing into another place, but the alternatives are even less palatable.
I tried with 0001 patch and can confirm that the wrong result issue shown in [1] is fixed.
explain (costs off, verbose) select min(i) from a; QUERY PLAN ----------------------------------------------------------- Gather Output: ($0) Workers Planned: 1 Params Evaluated: $0 <==== initplan params Single Copy: true InitPlan 1 (returns $0) -> Limit Output: a.i -> Index Only Scan using a_i_j_idx on public.a Output: a.i Index Cond: (a.i IS NOT NULL) -> Result Output: $0 (13 rows)
Now the Gather.initParam is filled and e89a71fb4 does its work to transmit the Params to workers.
So +1 to 0001 patch.
I'm still resistant to the idea of kluging EXPLAIN to the extent of hiding the EXPLAIN output changes. It wouldn't be that hard to do really, but I worry that such a kluge might hide real problems in future. So what I did in 0002 was to allow initPlans for an injected Gather only if debug_parallel_query = on, so that there will be a place for EXPLAIN to show them. Other than the changes in that area, 0002 is the same as the previous patch.
