Re: Very slow queries w/ NOT IN preparation (seems like a bug, test case)

Richard Huxton <dev(at)archonet(dot)com> writes:
> If you connect via psql and then (as root, in another terminal) do:
> ps auxw | grep postgres
> you should see the backend that corresponds to your psql connection.
> strace -p <pid>
> should then show system calls as they are executed (assuming you have it
> installed). Execute the explain, and see what is output.

> Mine flies past, but is composed almost entirely of "gettimeofday" calls
> (10,000 of them) apart from at the very end where we get some write and
> send/recv calls (to print the explain results). I've heard of some
> people having slow "gettimeofday" calls, but not on linux. On the other
> hand, that seems to be the main difference between strace output with
> "not in" compared to "in".

AFAICT Sergey is complaining about the speed of EXPLAIN, *not* EXPLAIN
ANALYZE. There'd only be a lot of gettimeofday calls in an EXPLAIN
ANALYZE test.

The whole thing doesn't make a lot of sense to me either. All the
slowdown explanations I can think of would apply as much or more to the
IN case...

regards, tom lane