Home > mailing lists

select count(*) is slow - Mailing list pgsql-performance

From	aditya desai
Subject	select count(*) is slow
Date	April 6, 2021 13:14:02
Msg-id	CAN0SRDFFCKvS0gv_bnn2Pg=G7eu7GAu+b0yeariVpuhQ_ewQXg@mail.gmail.com Whole thread Raw
Responses	Re: select count(*) is slow
List	pgsql-performance

Hi,

Below query takes 12 seconds. We have an index on postcode.

select count(*) from table where postcode >= '00420' AND postcode <= '00500'

index:

CREATE INDEX Table_i1
ON table USING btree
((postcode::numeric));

Table has 180,000 rows and the count is 150,000. Expectation is to run this query in 2-3 seconds(it takes 2 seconds in Oracle).

Here is a query plan:

"Aggregate (cost=622347.34..622347.35 rows=1 width=8) (actual time=12850.580..12850.580 rows=1 loops=1)"

" -> Bitmap Heap Scan on table (cost=413379.89..621681.38 rows=266383 width=0) (actual time=12645.656..12835.185 rows=209749 loops=1)"

" Recheck Cond: (((postcode)::text >= '00420'::text) AND ((postcode)::text <= '00500'::text))"

" Heap Blocks: exact=118286"

" -> Bitmap Index Scan on table_i4 (cost=0.00..413313.29 rows=266383 width=0) (actual time=12615.321..12615.321 rows=209982 loops=1)"

" Index Cond: (((postcode)::text >= '00420'::text) AND ((postcode)::text <= '00500'::text))"

"Planning Time: 0.191 ms"

"Execution Time: 12852.823 ms"

Regards,

Aditya.

From: hubert depesz lubaczewski
Date: 06 April 2021, 10:41:48
Subject: Re: Substitute for synonym in Oracle after migration to postgres

From: aditya desai
Date: 06 April 2021, 13:19:16
Subject: Re: Substitute for synonym in Oracle after migration to postgres