Re: Query performance changes significantly depending on limit value

2017-02-23 14:11 GMT+01:00 Rowan Seymour <rowanseymour(at)gmail(dot)com>:

> Hi guys
>
> I'm a bit stuck on a query that performs fantastically up to a certain
> limit value, after which the planner goes off in a completely different
> direction and performance gets dramatically worse. Am using Postgresql 9.3
>
> You can see all the relevant schemas at http://pastebin.com/PNEqw2id and
> in the test database there are 1,000,000 records in contacts_contact, and
> about half of those will match the subquery on values_value.
>
> The query in question is:
>
> SELECT "contacts_contact".* FROM "contacts_contact"
> INNER JOIN "contacts_contactgroup_contacts" ON ("contacts_contact"."id" =
> "contacts_contactgroup_contacts"."contact_id")
> WHERE ("contacts_contactgroup_contacts"."contactgroup_id" = 1
> AND "contacts_contact"."id" IN (
> SELECT U0."contact_id" FROM "values_value" U0 WHERE
> (U0."contact_field_id" = 1 AND UPPER(U0."string_value"::text) = UPPER('F'))
> )
> ) ORDER BY "contacts_contact"."id" DESC LIMIT 222;
>
> With that limit of 222, it performs like:
>
> Limit (cost=3.09..13256.36 rows=222 width=88) (actual time=0.122..3.358
> rows=222 loops=1)
> Buffers: shared hit=708 read=63
> -> Nested Loop (cost=3.09..59583.10 rows=998 width=88) (actual
> time=0.120..3.304 rows=222 loops=1)
> Buffers: shared hit=708 read=63
> -> Merge Semi Join (cost=2.65..51687.89 rows=2004 width=92)
> (actual time=0.103..1.968 rows=227 loops=1)
> Merge Cond: (contacts_contact.id = u0.contact_id)
> Buffers: shared hit=24 read=63
> -> Index Scan Backward using contacts_contact_pkey on
> contacts_contact (cost=0.42..41249.43 rows=1000000 width=88) (actual
> time=0.008..0.502 rows=1117 loops=1)
> Buffers: shared hit=22 read=2
> -> Index Scan using values_value_field_string_value_contact
> on values_value u0 (cost=0.43..7934.72 rows=2004 width=4) (actual
> time=0.086..0.857 rows=227 loops=1)
> Index Cond: ((contact_field_id = 1) AND
> (upper(string_value) = 'F'::text))
> Buffers: shared hit=2 read=61
> -> Index Only Scan using contacts_contactgroup_
> contacts_contactgroup_id_0f909f73_uniq on contacts_contactgroup_contacts
> (cost=0.43..3.93 rows=1 width=4) (actual time=0.005..0.005 rows=1
> loops=227)
> Index Cond: ((contactgroup_id = 1) AND (contact_id =
> contacts_contact.id))
> Heap Fetches: 0
> Buffers: shared hit=684
> Total runtime: 3.488 ms
>
> https://explain.depesz.com/s/iPPJ
>
> But if increase the limit to 223 then it performs like:
>
> Limit (cost=8785.68..13306.24 rows=223 width=88) (actual
> time=2685.830..2686.534 rows=223 loops=1)
> Buffers: shared hit=767648 read=86530
> -> Merge Join (cost=8785.68..29016.70 rows=998 width=88) (actual
> time=2685.828..2686.461 rows=223 loops=1)
> Merge Cond: (contacts_contact.id = contacts_contactgroup_
> contacts.contact_id)
> Buffers: shared hit=767648 read=86530
> -> Sort (cost=8784.44..8789.45 rows=2004 width=92) (actual
> time=2685.742..2685.804 rows=228 loops=1)
> Sort Key: contacts_contact.id
> Sort Method: quicksort Memory: 34327kB
> Buffers: shared hit=767648 read=86524
> -> Nested Loop (cost=6811.12..8674.53 rows=2004 width=92)
> (actual time=646.573..2417.291 rows=200412 loops=1)
>

There is pretty bad estimation probably due dependency between
contact_field_id = 1 and upper(string_value) = 'F'::text

The most simple solution is disable nested loop - set enable_nestloop to off

Regards

Pavel

> Buffers: shared hit=767648 read=86524
> -> HashAggregate (cost=6810.70..6813.14 rows=244
> width=4) (actual time=646.532..766.200 rows=200412 loops=1)
> Buffers: shared read=51417
> -> Bitmap Heap Scan on values_value u0
> (cost=60.98..6805.69 rows=2004 width=4) (actual time=92.016..433.709
> rows=200412 loops=1)
> Recheck Cond: ((contact_field_id = 1) AND
> (upper(string_value) = 'F'::text))
> Buffers: shared read=51417
> -> Bitmap Index Scan on
> values_value_field_string_value_contact (cost=0.00..60.47 rows=2004
> width=0) (actual time=70.647..70.647 rows=200412 loops=1)
> Index Cond: ((contact_field_id = 1)
> AND (upper(string_value) = 'F'::text))
> Buffers: shared read=770
> -> Index Scan using contacts_contact_pkey on
> contacts_contact (cost=0.42..7.62 rows=1 width=88) (actual
> time=0.007..0.007 rows=1 loops=200412)
> Index Cond: (id = u0.contact_id)
> Buffers: shared hit=767648 read=35107
> -> Index Only Scan Backward using contacts_contactgroup_
> contacts_contactgroup_id_0f909f73_uniq on contacts_contactgroup_contacts
> (cost=0.43..18967.29 rows=497992 width=4) (actual time=0.073..0.273
> rows=550 loops=1)
> Index Cond: (contactgroup_id = 1)
> Heap Fetches: 0
> Buffers: shared read=6
> Total runtime: 2695.301 ms
>
> https://explain.depesz.com/s/gXS
>
> I've tried running ANALYZE but that actually reduced the limit at which
> things get worse. Any insight into the reasoning of the query planner would
> be much appreciated.
>
> Thanks
>
> --
> *Rowan Seymour* | +260 964153686 <+260%2096%204153686> | @rowanseymour
>