Re: Erratically behaving query needs optimization

Yes, adding another index might help reduce the number of rows filtered
--> Rows Removed by Filter: 1308337

Also, make sure you run vacuum analyze on this query.

Regards,
Michael Vitale

Luís Roberto Weck wrote on 8/20/2019 10:58 AM:
> Em 20/08/2019 10:54, Barbu Paul - Gheorghe escreveu:
>> Hello,
>> I'm running "PostgreSQL 11.2, compiled by Visual C++ build 1914,
>> 64-bit" and I have a query that runs several times per user action
>> (9-10 times).
>> The query takes a long time to execute, specially at first, due to
>> cold caches I think, but the performance varies greatly during a run
>> of the application (while applying the said action by the user several
>> times).
>>
>> My tables are only getting bigger with time, not much DELETEs and even
>> less UPDATEs as far as I can tell.
>>
>> Problematic query:
>>
>> EXPLAIN (ANALYZE,BUFFERS)
>> SELECT DISTINCT ON (results.attribute_id) results.timestamp,
>> results.data FROM results
>>      JOIN scheduler_operation_executions ON
>> scheduler_operation_executions.id = results.operation_execution_id
>>      JOIN scheduler_task_executions ON scheduler_task_executions.id =
>> scheduler_operation_executions.task_execution_id
>> WHERE scheduler_task_executions.device_id = 97
>>      AND results.data <> '<NullData/>'
>>      AND results.data IS NOT NULL
>>      AND results.object_id = 1955
>>      AND results.attribute_id IN (4, 5) -- possibly a longer list here
>>      AND results.data_access_result = 'SUCCESS'
>> ORDER BY results.attribute_id, results.timestamp DESC
>> LIMIT 2 -- limit by the length of the attributes list
>>
>> In words: I want the latest (ORDER BY results.timestamp DESC) results
>> of a device (scheduler_task_executions.device_id = 97 - hence the
>> joins results -> scheduler_operation_executions ->
>> scheduler_task_executions)
>> for a given object and attributes with some additional constraints on
>> the data column. But I only want the latest attributes for which we
>> have results, hence the DISTINCT ON (results.attribute_id) and LIMIT.
>>
>> First run: https://explain.depesz.com/s/qh4C
>> Limit  (cost=157282.39..157290.29 rows=2 width=54) (actual
>> time=44068.166..44086.970 rows=2 loops=1)
>>    Buffers: shared hit=215928 read=85139
>>    ->  Unique  (cost=157282.39..157298.20 rows=4 width=54) (actual
>> time=44068.164..44069.301 rows=2 loops=1)
>>          Buffers: shared hit=215928 read=85139
>>          ->  Sort  (cost=157282.39..157290.29 rows=3162 width=54)
>> (actual time=44068.161..44068.464 rows=2052 loops=1)
>>                Sort Key: results.attribute_id, results."timestamp" DESC
>>                Sort Method: quicksort  Memory: 641kB
>>                Buffers: shared hit=215928 read=85139
>>                ->  Gather  (cost=62853.04..157098.57 rows=3162
>> width=54) (actual time=23518.745..44076.385 rows=4102 loops=1)
>>                      Workers Planned: 2
>>                      Workers Launched: 2
>>                      Buffers: shared hit=215928 read=85139
>>                      ->  Nested Loop  (cost=61853.04..155782.37
>> rows=1318 width=54) (actual time=23290.514..43832.223 rows=1367
>> loops=3)
>>                            Buffers: shared hit=215928 read=85139
>>                            ->  Parallel Hash Join
>> (cost=61852.61..143316.27 rows=24085 width=4) (actual
>> time=23271.275..40018.451 rows=19756 loops=3)
>>                                  Hash Cond:
>> (scheduler_operation_executions.task_execution_id =
>> scheduler_task_executions.id)
>>                                  Buffers: shared hit=6057 read=85139
>>                                  ->  Parallel Seq Scan on
>> scheduler_operation_executions  (cost=0.00..74945.82 rows=2482982
>> width=8) (actual time=7.575..15694.435 rows=1986887 loops=3)
>>                                        Buffers: shared hit=2996
>> read=47120
>>                                  ->  Parallel Hash
>> (cost=61652.25..61652.25 rows=16029 width=4) (actual
>> time=23253.337..23253.337 rows=13558 loops=3)
>>                                        Buckets: 65536  Batches: 1
>> Memory Usage: 2144kB
>>                                        Buffers: shared hit=2977
>> read=38019
>>                                        ->  Parallel Seq Scan on
>> scheduler_task_executions  (cost=0.00..61652.25 rows=16029 width=4)
>> (actual time=25.939..23222.174 rows=13558 loops=3)
>>                                              Filter: (device_id = 97)
>>                                              Rows Removed by Filter:
>> 1308337
>>                                              Buffers: shared hit=2977
>> read=38019
>>                            ->  Index Scan using
>> index_operation_execution_id_asc on results  (cost=0.43..0.51 rows=1
>> width=58) (actual time=0.191..0.191 rows=0 loops=59269)
>>                                  Index Cond: (operation_execution_id =
>> scheduler_operation_executions.id)
>>                                  Filter: ((data IS NOT NULL) AND (data
>> <> '<NullData/>'::text) AND (attribute_id = ANY ('{4,5}'::integer[]))
>> AND (object_id = 1955) AND (data_access_result = 'SUCCESS'::text))
>>                                  Rows Removed by Filter: 0
>>                                  Buffers: shared hit=209871
>> Planning Time: 29.295 ms
>> Execution Time: 44087.365 ms
>>
>>
>> Second run: https://explain.depesz.com/s/uy9f
>> Limit  (cost=157282.39..157290.29 rows=2 width=54) (actual
>> time=789.363..810.440 rows=2 loops=1)
>>    Buffers: shared hit=216312 read=84755
>>    ->  Unique  (cost=157282.39..157298.20 rows=4 width=54) (actual
>> time=789.361..789.535 rows=2 loops=1)
>>          Buffers: shared hit=216312 read=84755
>>          ->  Sort  (cost=157282.39..157290.29 rows=3162 width=54)
>> (actual time=789.361..789.418 rows=2052 loops=1)
>>                Sort Key: results.attribute_id, results."timestamp" DESC
>>                Sort Method: quicksort  Memory: 641kB
>>                Buffers: shared hit=216312 read=84755
>>                ->  Gather  (cost=62853.04..157098.57 rows=3162
>> width=54) (actual time=290.356..808.454 rows=4102 loops=1)
>>                      Workers Planned: 2
>>                      Workers Launched: 2
>>                      Buffers: shared hit=216312 read=84755
>>                      ->  Nested Loop  (cost=61853.04..155782.37
>> rows=1318 width=54) (actual time=238.313..735.472 rows=1367 loops=3)
>>                            Buffers: shared hit=216312 read=84755
>>                            ->  Parallel Hash Join
>> (cost=61852.61..143316.27 rows=24085 width=4) (actual
>> time=237.966..677.975 rows=19756 loops=3)
>>                                  Hash Cond:
>> (scheduler_operation_executions.task_execution_id =
>> scheduler_task_executions.id)
>>                                  Buffers: shared hit=6441 read=84755
>>                                  ->  Parallel Seq Scan on
>> scheduler_operation_executions  (cost=0.00..74945.82 rows=2482982
>> width=8) (actual time=0.117..234.279 rows=1986887 loops=3)
>>                                        Buffers: shared hit=3188
>> read=46928
>>                                  ->  Parallel Hash
>> (cost=61652.25..61652.25 rows=16029 width=4) (actual
>> time=236.631..236.631 rows=13558 loops=3)
>>                                        Buckets: 65536  Batches: 1
>> Memory Usage: 2144kB
>>                                        Buffers: shared hit=3169
>> read=37827
>>                                        ->  Parallel Seq Scan on
>> scheduler_task_executions  (cost=0.00..61652.25 rows=16029 width=4)
>> (actual time=0.132..232.758 rows=13558 loops=3)
>>                                              Filter: (device_id = 97)
>>                                              Rows Removed by Filter:
>> 1308337
>>                                              Buffers: shared hit=3169
>> read=37827
>>                            ->  Index Scan using
>> index_operation_execution_id_asc on results  (cost=0.43..0.51 rows=1
>> width=58) (actual time=0.003..0.003 rows=0 loops=59269)
>>                                  Index Cond: (operation_execution_id =
>> scheduler_operation_executions.id)
>>                                  Filter: ((data IS NOT NULL) AND (data
>> <> '<NullData/>'::text) AND (attribute_id = ANY ('{4,5}'::integer[]))
>> AND (object_id = 1955) AND (data_access_result = 'SUCCESS'::text))
>>                                  Rows Removed by Filter: 0
>>                                  Buffers: shared hit=209871
>> Planning Time: 1.787 ms
>> Execution Time: 810.634 ms
>>
>> You can see that the second run takes less than one second to run...
>> which is 43 seconds better than the first try, just by re-running the
>> query.
>> Other runs take maybe 1s, 3s, still a long time.
>>
>> How can I improve it to be consistently fast (is it possible to get to
>> several milliseconds?)?
>> What I don't really understand is why the nested loop has 3 loops
>> (three joined tables)?
>> And why does the first index scan indicate ~60k loops? And does it
>> really work? It doesn't seem to filter out any rows.
>>
>> Should I add an index only on (attribute_id, object_id)? And maybe
>> data_access_result?
>> Does it make sens to add it on a text column (results.data)?
>>
>> My tables:
>> https://gist.githubusercontent.com/paulbarbu/0d36271d710349d8fb6102d9a466bb54/raw/7a6946ba7c2adec5b87ed90f343f1aff37432d21/gistfile1.txt
>>
>>
>> As you can see from the gist the foreign keys are indexed. Other
>> indices were added to speed up other queries.
>> Other relevant information (my tables have 3+ millions of rows, not
>> very big I think?), additional info with regards to size also included
>> below.
>> This query has poor performance on two PCs (both running off of HDDs)
>> so I think it has more to do with my indices and query than Postgres
>> config & hardware, will post those if necessary.
>>
>>
>> Size info:
>> SELECT relname, relpages, reltuples, relallvisible, relkind, relnatts,
>> relhassubclass, reloptions, pg_table_size(oid) FROM pg_class WHERE
>> relname IN ('results', 'scheduler_operation_executions',
>> 'scheduler_task_executions');
>> -[ RECORD 1 ]--+-------------------------------
>> relname        | results
>> relpages       | 65922
>> reltuples      | 3.17104e+06
>> relallvisible  | 65922
>> relkind        | r
>> relnatts       | 9
>> relhassubclass | f
>> reloptions     |
>> pg_table_size  | 588791808
>> -[ RECORD 2 ]--+-------------------------------
>> relname        | scheduler_operation_executions
>> relpages       | 50116
>> reltuples      | 5.95916e+06
>> relallvisible  | 50116
>> relkind        | r
>> relnatts       | 8
>> relhassubclass | f
>> reloptions     |
>> pg_table_size  | 410697728
>> -[ RECORD 3 ]--+-------------------------------
>> relname        | scheduler_task_executions
>> relpages       | 40996
>> reltuples      | 3.966e+06
>> relallvisible  | 40996
>> relkind        | r
>> relnatts       | 12
>> relhassubclass | f
>> reloptions     |
>> pg_table_size  | 335970304
>>
>> Thanks for your time!
>>
>> --
>> Barbu Paul - Gheorghe
>>
> Can you create an index on scheduler_task_executions.device_id and run
> it again?
>
>