Re: Same query 10000x More Time

On Thu, 6 Jan 2022 at 13:13, Avi Weinberg <AviW(at)gilat(dot)com> wrote:

> Hi
>
>
>
> I have postgres_fdw table called tbl_link. The source table is 2.5 GB in
> size with 122 lines (some lines has 70MB bytea column, but not the ones I
> select in the example)
>
> I noticed that when I put the specific ids in the list "where id in
> (140,144,148)" it works fast (few ms), but when I put the same list as
> select "where id in (select 140 as id union select 144 union select 148)"
> it takes 50 seconds. This select union is just for the example, I
> obviously have a different select (which by itself takes few ms but cause
> the whole insert query to take 10000x more time)
>
>
>
> Why is that? How can I still use regular select and still get reasonable
> response time?
>
>
>
> Thanks
>
>
>

couple of things:
PostgreSQL: Documentation: 14: F.35. postgres_fdw
<https://www.postgresql.org/docs/current/postgres-fdw.html>
<https://www.postgresql.org/docs/current/postgres-fdw.html>when you set
your foreign server what are your
use_remote_estimate
fetch_size
params for the foreign server.

you need to know there are certain restrictions on what gets pushed down to
the remote server
i generally use postgres/postgres_fdw.sql at master · postgres/postgres
(github.com)
<https://github.com/postgres/postgres/blob/master/contrib/postgres_fdw/sql/postgres_fdw.sql>
as
a reference
if you predicates are not pushed down, it will bring all the rows from the
foreign server to your local server (and fetch_size value and network io
will add to delay)
and given you used select * , it will be a lot of io, so maybe restrict
only to columns needed after being filtered would help.

you can try by running
explain (verbose,analyze) query and then also enabling log_statement =
'all' / log_min_duration_statement = 0
on the foreign server to see the actual plan for the foreign scan.

That might help in trouble shooting.

as always, i have little production exposure. If i am wrong, i can be
corrected.