On Wed, Jul 12, 2017 at 1:50 AM, Jeff Janes <jeff.janes@gmail.com> wrote: > On Mon, Jul 10, 2017 at 9:51 PM, Dilip Kumar <dilipbalaut@gmail.com> wrote: >> >> So because of this high projection cost the seqpath and parallel path >> both have fuzzily same cost but seqpath is winning because it's >> parallel safe. > > > I think you are correct. However, unless parallel_tuple_cost is set very > low, apply_projection_to_path never gets called with the Gather path as an > argument. It gets ruled out at some earlier stage, presumably because it > assumes the projection step cannot make it win if it is already behind by > enough. >
I think that is genuine because tuple communication cost is very high.
Sorry, I don't know which you think is genuine, the early pruning or my complaint about the early pruning.
I agree that the communication cost is high, which is why I don't want to have to set parellel_tuple_cost very low. For example, to get the benefit of your patch, I have to set parellel_tuple_cost to 0.0049 or less (in my real-world case, not the dummy test case I posted, although the number are around the same for that one too). But with a setting that low, all kinds of other things also start using parallel plans, even if they don't benefit from them and are harmed.
I realize we need to do some aggressive pruning to avoid an exponential explosion in planning time, but in this case it has some rather unfortunate consequences. I wanted to explore it, but I can't figure out where this particular pruning is taking place.
By the time we get to planner.c line 1787, current_rel->pathlist already does not contain the parallel plan if parellel_tuple_cost >= 0.0050, so the pruning is happening earlier than that.
If your table is reasonable large then you might want to try by increasing parallel workers (Alter Table ... Set (parallel_workers = ..))
Setting parallel_workers to 8 changes the threshold for the parallel to even be considered from parellel_tuple_cost <= 0.0049 to <= 0.0076. So it is going in the correct direction, but not by enough to matter.
