Hi Pavel. That suggestion gets me as far as LIMIT 694 with the fast plan
then things get slow again. This is now what happens at LIMIT 695:
Limit (cost=35945.78..50034.52 rows=695 width=88) (actual
time=12852.580..12854.382 rows=695 loops=1)
Buffers: shared hit=6 read=66689
-> Merge Join (cost=35945.78..56176.80 rows=998 width=88) (actual
time=12852.577..12854.271 rows=695 loops=1)
Merge Cond: (contacts_contact.id =
contacts_contactgroup_contacts.contact_id)
Buffers: shared hit=6 read=66689
-> Sort (cost=35944.53..35949.54 rows=2004 width=92) (actual
time=12852.486..12852.577 rows=710 loops=1)
Sort Key: contacts_contact.id
Sort Method: quicksort Memory: 34327kB
Buffers: shared hit=6 read=66677
-> Hash Join (cost=6816.19..35834.63 rows=2004 width=92)
(actual time=721.293..12591.204 rows=200412 loops=1)
Hash Cond: (contacts_contact.id = u0.contact_id)
Buffers: shared hit=6 read=66677
-> Seq Scan on contacts_contact (cost=0.00..25266.00
rows=1000000 width=88) (actual time=0.003..267.258 rows=1000000 loops=1)
Buffers: shared hit=1 read=15265
-> Hash (cost=6813.14..6813.14 rows=244 width=4)
(actual time=714.373..714.373 rows=200412 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 7046kB
Buffers: shared hit=5 read=51412
-> HashAggregate (cost=6810.70..6813.14
rows=244 width=4) (actual time=561.099..644.822 rows=200412 loops=1)
Buffers: shared hit=5 read=51412
-> Bitmap Heap Scan on values_value u0
(cost=60.98..6805.69 rows=2004 width=4) (actual time=75.410..364.976
rows=200412 loops=1)
Recheck Cond: ((contact_field_id = 1)
AND (upper(string_value) = 'F'::text))
Buffers: shared hit=5 read=51412
-> Bitmap Index Scan on
values_value_field_string_value_contact (cost=0.00..60.47 rows=2004
width=0) (actual time=57.642..57.642 rows=200412 loops=1)
Index Cond: ((contact_field_id
= 1) AND (upper(string_value) = 'F'::text))
Buffers: shared hit=5 read=765
-> 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=0.080..0.651 rows=1707 loops=1)
Index Cond: (contactgroup_id = 1)
Heap Fetches: 0
Buffers: shared read=12
Total runtime: 12863.938 ms
https://explain.depesz.com/s/nfw1
Can you explain a bit more about what you mean about " dependency between
contact_field_id = 1 and upper(string_value) = 'F'::text"?
Btw I created the index values_value_field_string_value_contact as
CREATE INDEX values_value_field_string_value_contact
ON values_value(contact_field_id, UPPER(string_value), contact_id DESC)
WHERE contact_field_id IS NOT NULL;
I'm not sure why it needs the contact_id column but without it the planner
picks a slow approach for even smaller LIMIT values.
On 23 February 2017 at 15:32, Pavel Stehule <pavel.steh...@gmail.com> wrote:
>
>
> 2017-02-23 14:11 GMT+01:00 Rowan Seymour <rowanseym...@gmail.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
>>
>
>
--
*Rowan Seymour* | +260 964153686 | @rowanseymour
