Re: Query running slow for only one specific id. (Postgres 9.3) version

From: Sheena, Prabhjot [mailto:Prabhjot(dot)Singh(at)classmates(dot)com]
Sent: Friday, June 05, 2015 2:38 PM
To: Igor Neyman; pgsql-general(at)postgresql(dot)org; pgsql-performance(at)postgresql(dot)org
Subject: RE: Query running slow for only one specific id. (Postgres 9.3) version

When I run vacuum analyze it fixes the problem but after 1 or 2 days the problem comes back

Here is the table structure

Column | Type | Modifiers | Storage | Stats target | Description
------------------+-----------------------------+----------------------------------------------------------------------+---------+--------------+-------------
response_id | integer | not null default nextval('btdt_responses_response_id_seq'::regclass) | plain | |
registration_id | bigint | not null | plain | |
btdt_id | integer | not null | plain | |
response | integer | not null | plain | |
creation_date | timestamp without time zone | not null default now() | plain | |
last_update_date | timestamp without time zone | not null default now() | plain | |
Indexes:
"btdt_responses_pkey" PRIMARY KEY, btree (response_id)
"btdt_responses_u2" UNIQUE, btree (registration_id, btdt_id)
"btdt_responses_n1" btree (btdt_id)
"btdt_responses_n2" btree (btdt_id, response)
"btdt_responses_n4" btree (creation_date)
"btdt_responses_n5" btree (last_update_date)
"btdt_responses_n6" btree (btdt_id, last_update_date)
Foreign-key constraints:
"btdt_responses_btdt_id_fkey" FOREIGN KEY (btdt_id) REFERENCES btdt_items(btdt_id)
"btdt_responses_fk1" FOREIGN KEY (btdt_id) REFERENCES btdt_items(btdt_id)
Has OIDs: no
Options: autovacuum_enabled=true, autovacuum_vacuum_scale_factor=0.02, autovacuum_analyze_scale_factor=0.02

Thanks

From: Igor Neyman [mailto:ineyman(at)perceptron(dot)com]
Sent: Friday, June 5, 2015 11:06 AM
To: Sheena, Prabhjot; pgsql-general(at)postgresql(dot)org<mailto:pgsql-general(at)postgresql(dot)org>; pgsql-performance(at)postgresql(dot)org<mailto:pgsql-performance(at)postgresql(dot)org>
Subject: RE: Query running slow for only one specific id. (Postgres 9.3) version

From: pgsql-performance-owner(at)postgresql(dot)org<mailto:pgsql-performance-owner(at)postgresql(dot)org> [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Sheena, Prabhjot
Sent: Friday, June 05, 2015 1:55 PM
To: pgsql-general(at)postgresql(dot)org<mailto:pgsql-general(at)postgresql(dot)org>; pgsql-performance(at)postgresql(dot)org<mailto:pgsql-performance(at)postgresql(dot)org>
Subject: [PERFORM] Query running slow for only one specific id. (Postgres 9.3) version

Postgresql 9.3 Version

Guys
Here is the issue that I'm facing for couple of weeks now. I have table (size 7GB)

If I run this query with this specific registration id it is using the wrong execution plan and takes more than a minute to complete. Total number of rows for this registration_id is only 414 in this table

explain analyze SELECT max(last_update_date) AS last_update_date FROM btdt_responses WHERE registration_id = 8718704208 AND response != 4;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Result (cost=2902.98..2903.01 rows=1 width=0) (actual time=86910.730..86910.731 rows=1 loops=1)
InitPlan 1 (returns $0)
-> Limit (cost=0.57..2902.98 rows=1 width=8) (actual time=86910.725..86910.725 rows=1 loops=1)
-> Index Scan Backward using btdt_responses_n5 on btdt_responses (cost=0.57..6425932.41 rows=2214 width=8) (actual time=86910.723..86910.723 rows=1 loops=1)
Index Cond: (last_update_date IS NOT NULL)
Filter: ((response <> 4) AND (registration_id = 8718704208::bigint))
Rows Removed by Filter: 52145434
Total runtime: 86910.766 ms

Same query with any other registration id will come back in milli seconds

explain analyze SELECT max(last_update_date) AS last_update_date FROM btdt_responses WHERE registration_id = 8688546267 AND response != 4;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=529.75..529.78 rows=1 width=8) (actual time=19.723..19.723 rows=1 loops=1)
-> Index Scan using btdt_responses_u2 on btdt_responses (cost=0.57..529.45 rows=119 width=8) (actual time=0.097..19.689 rows=72 loops=1)
Index Cond: (registration_id = 8688546267::bigint)
Filter: (response <> 4)
Rows Removed by Filter: 22
Total runtime: 19.769 ms

Please let me know what I can do to fix this issue.

Thanks

Not enough info.
Table structure? Is registration_id - PK? If not, what is the distribution of the values for this table?
When was it analyzed last time? M.b. you need to increase statistics target for this table:

Index Scan Backward using btdt_responses_n5 on btdt_responses (cost=0.57..6425932.41 rows=2214 width=8) (actual time=86910.723..86910.723 rows=1 loops=1)

It expects 2214 records while really getting only 1.

Regards,
Igor Neyman

Do you have autovacuum running?
If yes, maybe it's not aggressive enough and you need to adjust its parameters.

Regards,
Igor Neyman