Re: Extremely slow count (simple query, with index)

Hi Marco,

Since you said approximates would be good enough, there are two ways to 
do that.  Query pg_class.reltuples or pg_stat_user_tables.n_live_tup. 
Personally, I prefer the pg_stat_user tables since it is more current 
than pg_class table, unless you run ANALYZE on your target table before 
querying pg_class table.  Then of course you get results in a few 
milliseconds since you do not incur the tablescan cost of selecting 
directly from the target table.

Regards,
Michael Vitale


Marco Colli wrote on 8/22/2019 8:44 AM:
> Hello!
>
> Any help would be greatly appreciated.
> I need to run these simple queries on a table with millions of rows:
>
> ```
> SELECT COUNT(*) FROM "subscriptions" WHERE 
> "subscriptions"."project_id" = 123;
> ```
>
> ```
> SELECT COUNT(*) FROM "subscriptions" WHERE 
> "subscriptions"."project_id" = 123 AND "subscriptions"."trashed_at" IS 
> NULL;
> ```
>
> The count result for both queries, for project 123, is about 5M.
>
> I have an index in place on `project_id`, and also another index on 
> `(project_id, trashed_at)`:
>
> ```
> "index_subscriptions_on_project_id_and_created_at" btree (project_id, 
> created_at DESC)
> "index_subscriptions_on_project_id_and_trashed_at" btree (project_id, 
> trashed_at DESC)
> ```
>
> The problem is that both queries are extremely slow and take about 17s 
> each.
>
> These are the results of `EXPLAIN ANALIZE`:
>
>
> ```
> QUERY PLAN
> --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>  Aggregate  (cost=2068127.29..2068127.30 rows=1 width=0) (actual 
> time=17342.420..17342.420 rows=1 loops=1)
>    ->  Bitmap Heap Scan on subscriptions  (cost=199573.94..2055635.23 
> rows=4996823 width=0) (actual time=1666.409..16855.610 rows=4994254 
> loops=1)
>        Recheck Cond: (project_id = 123)
>          Rows Removed by Index Recheck: 23746378
>          Heap Blocks: exact=131205 lossy=1480411
>          ->  Bitmap Index Scan on 
> index_subscriptions_on_project_id_and_trashed_at 
>  (cost=0.00..198324.74 rows=4996823 width=0) (actual 
> time=1582.717..1582.717 rows=4994877 loops=1)
>                Index Cond: (project_id = 123)
>  Planning time: 0.090 ms
>  Execution time: 17344.182 ms
> (9 rows)
> ```
>
>
> ```
>     QUERY PLAN
> --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>  Aggregate  (cost=2047881.69..2047881.70 rows=1 width=0) (actual 
> time=17557.218..17557.218 rows=1 loops=1)
>    ->  Bitmap Heap Scan on subscriptions  (cost=187953.70..2036810.19 
> rows=4428599 width=0) (actual time=1644.966..17078.378 rows=4994130 
> loops=1)
>        Recheck Cond: ((project_id = 123) AND (trashed_at IS NULL))
>        Rows Removed by Index Recheck: 23746273
>          Heap Blocks: exact=131144 lossy=1480409
>          ->  Bitmap Index Scan on 
> index_subscriptions_on_project_id_and_trashed_at 
>  (cost=0.00..186846.55 rows=4428599 width=0) (actual 
> time=1566.163..1566.163 rows=4994749 loops=1)
>  Index Cond: ((project_id = 123) AND (trashed_at IS NULL))
>  Planning time: 0.084 ms
>  Execution time: 17558.522 ms
> (9 rows)
> ```
>
> What is the problem?
> What can I do to improve the performance (i.e. count in a few seconds)?
>
> I have also tried to increase work_mem from 16MB to 128MB without any 
> improvement.
> Even an approximate count would be enough.
> Postgresql v9.5
>