On Fri, Nov 20, 2015 at 7:06 PM, Robert Haas <robertmhaas@gmail.com> wrote: > > On Fri, Nov 20, 2015 at 12:45 AM, Amit Kapila <amit.kapila16@gmail.com> wrote: > > Okay, but I think that's not what I am talking about. I am talking about > > below code in cost_seqscan: > > > > - if (nworkers > 0) > > > > - run_cost = run_cost / (nworkers + 0.5); > > > > + if (path->parallel_degree > 0) > > > > + run_cost = run_cost / (path->parallel_degree + 0.5); > > > > > > It will consider 50% of master backends effort for scan of each child > > relation, > > does that look correct to you? Wouldn't 50% of master backends effort be > > considered to scan all the child relations? > > In the code you originally wrote, you were adding 1 there rather than > 0.5. That meant you were expecting the leader to do as much work as > each of its workers, which is clearly a bad estimate, because the > leader also has to do the work of gathering tuples from the workers. > 0.5 might not be the right value, but it's surely better than 1. >
Without this patch, that 0.5 (or 50% of leaders effort) is considered for
Gather node irrespective of the number of workers or other factors, but
I think with Patch that is no longer true and that's what I am worrying
