Fd Habash <fmhab...@gmail.com> writes:
> Based on my research in the forums and Google , it is described in multiple
> places that ‘select count(*)’ is expected to be slow in Postgres because of
> the MVCC controls imposed upon the query leading a table scan. Also, the
> elapsed time increase linearly with table size.
> However, I do not know if elapsed time I’m getting is to be expected.
> Table reltuples in pg_class = 2,266,649,344 (pretty close)
> Query = select count(*) from jim.sttyations ;
> Elapsed time (ET) = 18.5 hrs
That's pretty awful. My recollection is that in recent PG releases,
SELECT COUNT(*) runs at something on the order of 100ns/row given an
all-in-memory table. Evidently you're rather badly I/O bound.
> This is an Aurora cluster running on r4.2xlarge (8 vCPU, 61g).
Don't know much about Aurora, but I wonder whether you paid for
guaranteed (provisioned) IOPS, and if so what service level.
> refpep-> select count(*) from jim.sttyations;
> QUERY PLAN
>
> ------------------------------------------------------------------------------------------------------------------
> Aggregate (cost=73451291.77..73451291.78 rows=1 width=8)
> Output: count(*)
> -> Index Only Scan using stty_indx_fk03 on jim.sttyations
> (cost=0.58..67784668.41 rows=2266649344 width=0)
> Output: vsr_number
> (4 rows)
Oh, hmm ... the 100ns figure I mentioned was for a seqscan. IOS
could be a lot worse for a number of reasons, foremost being that
if the table isn't mostly all-visible then it'd involve a lot of
random heap access. It might be interesting to try forcing a
seqscan plan (see enable_indexscan).
regards, tom lane
