you can play around various `enable_*` flags to see if disabling any of these will *maybe* yield the plan you were expecting, and then check the costs in EXPLAIN to see if the optimiser also thinks this plan is cheaper. On Mon, Mar 22, 2021 at 6:29 PM Chris Stephens <cstephens16@xxxxxxxxx> wrote: > > we are but i was hoping to get a better understanding of where the optimizer is going wrong and what i can do about it. > > chris > > > On Mon, Mar 22, 2021 at 9:54 AM Laurenz Albe <laurenz.albe@xxxxxxxxxxx> wrote: >> >> On Mon, 2021-03-22 at 08:10 -0500, Chris Stephens wrote: >> > The following SQL takes ~25 seconds to run. I'm relatively new to postgres >> > but the execution plan (https://explain.depesz.com/s/N4oR) looks like it's >> > materializing the entire EXISTS subquery for each row returned by the rest >> > of the query before probing for plate_384_id existence. postgres is >> > choosing sequential scans on sample_plate_384 and test_result when suitable, >> > efficient indexes exist. a re-written query produces a much better plan >> > (https://explain.depesz.com/s/zXJ6). Executing the EXISTS portion of the >> > query with an explicit PLATE_384_ID yields the execution plan we want as >> > well (https://explain.depesz.com/s/3QAK). unnesting the EXISTS and adding >> > a DISTINCT on the result also yields a better plan. >> >> Great! Then use one of the rewritten queries. >> >> Yours, >> Laurenz Albe >> -- >> Cybertec | https://www.cybertec-postgresql.com >>
