Re: [PERFORM] Query performance changes significantly depending onlimit value

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)

                    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 | @rowanseymour