Hello folks, Thank you very much for analysis and suggested - there is a lot to learn here. I just tried UNION queries and got following error:
ERROR: FOR UPDATE is not allowed with UNION/INTERSECT/EXCEPT I made a table dump for anyone who wants to give it a spin https://app.box.com/s/464b12glmlk5o4gvzz7krc4c8s2fxlwr and here is the gist for the original commands https://gist.github.com/lessless/33215d0c147645db721e74e07498ac53 On Wed, Jun 28, 2017 at 8:10 PM, Brad DeJong <brad.dej...@infor.com> wrote: > > > On 2017-06-28, Pavel Stehule wrote ... > > On 2017-06-28, Yevhenii Kurtov wrote ... > >> On 2017-06-28, Pavel Stehule wrote ... > >>> On 2017-06-28, Yevhenii Kurtov wrote ... > >>>> We have a query that is run almost each second and it's very > important to squeeze every other ms out of it. The query is: > >>>> ... > >>>> I added following index: CREATE INDEX ON campaign_jobs(id, status, > failed_at, started_at, priority DESC, times_failed); > >>>> ... > >>> There are few issues > >>> a) parametrized LIMIT > >>> b) complex predicate with lot of OR > >>> c) slow external sort > >>> > >>> b) signalize maybe some strange in design .. try to replace "OR" by > "UNION" query > >>> c) if you can and you have good enough memory .. try to increase > work_mem .. maybe 20MB > >>> > >>> if you change query to union queries, then you can use conditional > indexes > >>> > >>> create index(id) where status = 0; > >>> create index(failed_at) where status = 2; > >>> create index(started_at) where status = 1; > >> > >> Can you please give a tip how to rewrite the query with UNION clause? > > > > SELECT c0."id" FROM "campaign_jobs" AS c0 > > WHERE (((c0."status" = $1) AND NOT (c0."id" = ANY($2)))) > > UNION SELECT c0."id" FROM "campaign_jobs" AS c0 > > WHERE ((c0."status" = $3) AND (c0."failed_at" > $4)) > > UNION SELECT c0."id" FROM "campaign_jobs" AS c0 > > WHERE ((c0."status" = $5) AND (c0."started_at" < $6)) > > ORDER BY c0."priority" DESC, c0."times_failed" > > LIMIT $7 > > FOR UPDATE SKIP LOCKED > > > Normally (at least for developers I've worked with), that kind of query > structure is used when the "status" values don't overlap and don't change > from query to query. Judging from Pavel's suggested conditional indexes > (i.e. "where status = <constant>"), he also thinks that is likely. > > Give the optimizer that information so that it can use it. Assuming $1 = 0 > and $3 = 2 and $5 = 1, substitute literals. Substitute literal for $7 in > limit. Push order by and limit to each branch of the union all (or does > Postgres figure that out automatically?) Replace union with union all (not > sure about Postgres, but allows other dbms to avoid sorting and merging > result sets to eliminate duplicates). (Use of UNION ALL assumes that "id" > is unique across rows as implied by only "id" being selected with FOR > UPDATE. If multiple rows can have the same "id", then use UNION to > eliminate the duplicates.) > > SELECT "id" FROM "campaign_jobs" WHERE "status" = 0 AND NOT "id" = ANY($1) > UNION ALL > SELECT "id" FROM "campaign_jobs" WHERE "status" = 2 AND "failed_at" > $2 > UNION ALL > SELECT "id" FROM "campaign_jobs" WHERE "status" = 1 AND "started_at" < $3 > ORDER BY "priority" DESC, "times_failed" > LIMIT 100 > FOR UPDATE SKIP LOCKED > > > Another thing that you could try is to push the ORDER BY and LIMIT to the > branches of the UNION (or does Postgres figure that out automatically?) and > use slightly different indexes. This may not make sense for all the > branches but one nice thing about UNION is that each branch can be tweaked > independently. Also, there are probably unmentioned functional dependencies > that you can use to reduce the index size and/or improve your match rate. > Example - if status = 1 means that the campaign_job has started but not > failed or completed, then you may know that started_at is set, but > failed_at and ended_at are null. The < comparison in and of itself implies > that only rows where "started_at" is not null will match the condition. > > SELECT c0."id" FROM "campaign_jobs" AS c0 WHERE (((c0."status" = 0) AND > NOT (c0."id" = ANY($1)))) ORDER BY c0."priority" DESC, c0."times_failed" > LIMIT 100 > UNION ALL > SELECT c0."id" FROM "campaign_jobs" AS c0 WHERE ((c0."status" = 2) AND > (c0."failed_at" > $2)) ORDER BY c0."priority" DESC, c0."times_failed" LIMIT > 100 > UNION ALL > SELECT c0."id" FROM "campaign_jobs" AS c0 WHERE ((c0."status" = 1) AND > (c0."started_at" < $3)) ORDER BY c0."priority" DESC, c0."times_failed" > LIMIT 100 > ORDER BY c0."priority" DESC, c0."times_failed" > LIMIT 100 > FOR UPDATE SKIP LOCKED > > Including the "priority", "times_failed" and "id" columns in the indexes > along with "failed_at"/"started_at" allows the optimizer to do index only > scans. (May still have to do random I/O to the data page to determine tuple > version visibility but I don't think that can be eliminated.) > > create index ... ("priority" desc, "times_failed", "id") > where "status" = 0; > create index ... ("priority" desc, "times_failed", "id", "failed_at") > where "status" = 2 and "failed_at" is not null; > create index ... ("priority" desc, "times_failed", "id", "started_at") > where "status" = 1 and "started_at" is not null; -- and ended_at is null > and ... > > > I'm assuming that the optimizer knows that "where status = 1 and > started_at < $3" implies "and started_at is not null" and will consider the > conditional index. If not, then the "and started_at is not null" needs to > be explicit. > > -- > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance >
