On Thu, 6 Jan 2022 at 13:13, Avi Weinberg <a...@gilat.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 ยท 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.
