2017-06-29 7:17 GMT+02:00 Yevhenii Kurtov <yevhenii.kur...@gmail.com>:
> 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
>
it is sad :(
maybe bitmap index scan can work
postgres=# create table test(id int, started date, failed date, status int);
CREATE TABLE
postgres=# create index on test(id) where status = 0;
CREATE INDEX
postgres=# create index on test(started) where status = 1;
CREATE INDEX
postgres=# create index on test(failed ) where status = 2;
CREATE INDEX
postgres=# explain select id from test where (status = 0 and id in
(1,2,3,4,5)) or (status = 1 and started < current_date) or (status = 2 and
failed > current_date);
┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│
QUERY PLAN
╞════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════
│ Bitmap Heap Scan on test (cost=12.93..22.50 rows=6 width=4)
│ Recheck Cond: (((id = ANY ('{1,2,3,4,5}'::integer[])) AND (status = 0))
OR ((started < CURRENT_DATE) AND (status = 1)) OR ((faile
│ Filter: (((status = 0) AND (id = ANY ('{1,2,3,4,5}'::integer[]))) OR
((status = 1) AND (started < CURRENT_DATE)) OR ((status = 2)
│ -> BitmapOr (cost=12.93..12.93 rows=6 width=0)
│ -> Bitmap Index Scan on test_id_idx (cost=0.00..4.66 rows=1
width=0)
│ Index Cond: (id = ANY ('{1,2,3,4,5}'::integer[]))
│ -> Bitmap Index Scan on test_started_idx (cost=0.00..4.13
rows=3 width=0)
│ Index Cond: (started < CURRENT_DATE)
│ -> Bitmap Index Scan on test_failed_idx (cost=0.00..4.13 rows=3
width=0)
│ Index Cond: (failed > CURRENT_DATE)
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
(10 rows)
>
> 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.
>>
>> --
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>
>
