> > This looks like a costing bug. The estimated cost of sorting 416,667 > estimated tuples in one parallel worker is almost identical to the estimated > cost of sorting 1,000,000 tuples when parallelism is turned off. Adding > some logging to the cost_sort function, it looks like the limit does not get > sent down for the parallel estimate: > > NOTICE: JJ cost_sort tuples 1000000.000000, limit 61.000000, sort_mem 65536 > NOTICE: JJ cost_sort tuples 416667.000000, limit -1.000000, sort_mem 65536 > > So the LIMIT gets passed down for the execution step, but not for the > planning step.
Oh, well spotted. I was looking in the wrong place. Maybe the fix is as simple as the attached? It certainly helps in the cases I tested, including with wider tables.
I had hit on the same change. And was also surprised that it was located where it was. With the change, it uses the parallel plan all the way down to LIMIT 1.
With the patch, it still satisfies make check, so if it introduces errors they are subtle ones. If we can't actually do this and it needs to stay -1, then I think we need a comment to explain why.