On Fri, Dec 29, 2023 at 10:25 AM Jeff Janes <jeff.ja...@gmail.com> wrote:
> > > On Thu, Dec 28, 2023 at 7:47 AM mohini mane <mohini.andr...@gmail.com> > wrote: > >> Thank you for your response !! >> I am experimenting with SQL query performance for SELECT queries on large >> tables and I observed that changing/increasing the degree of parallel hint >> doesn't give the expected performance improvement. >> > > But you still have addressed the fact that PostgreSQL *does not have > planner hints*. > > Are you using some nonstandard extension, or nonstandard fork? > * >> I am using pg_hint_plan extension to enforce the parallel execution > of specific table .* > * postgres=# load 'pg_hint_plan';* * LOAD* > I have executed the SELECT query with 2,4 & 6 parallel degree however >> every time only 4 workers launched & there was a slight increase in >> Execution time as well, >> > > Adding an ignored comment to your SQL would not be expected to do > anything. So it is not surprising that it does not do anything about > the number of workers launched. It is just a comment. A note to the human > who is reading the code. > * >> As I am using ph_hint_plan extension so as expected hints should not > get ignored by the optimizer .* > >> why there is an increase in execution time with parallel degree 6 as >> compared to 2 or 4? >> > > Those small changes seem to be perfectly compatible with random noise. > You would need to repeat them dozens of times in random order, and then do > a statistical test to convince me otherwise. > * >> I am expecting desired number of parallel workers should get > allocated as VM has sufficient vCores [16] and with needed session > parameters > [parallel_tuple_cost=0.1,max_parallel_workers_per_gather=6,**max_parallel_workers=8 > and I am using parallel hints like this : * */*+ PARALLEL(A 5 hard) */ > so 5 worker processes should launched this is not happening]* > >>