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

Dear ,

Create the below indexes and try it !!!

create index ind_ subscriptions_ project_id on
"subscriptions"("project_id")
Where "project_id"= 1

create index ind_ subscriptions_ trashed_at on "subscriptions"("
trashed_at ")
Where "trashed_at" is null

On Thu, Aug 22, 2019 at 6:36 PM Marco Colli <collimarco91(at)gmail(dot)com> wrote:

> 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
>
>

--
*Regards,*
*Ravikumar S,*
*Ph: 8106741263*