Re: Parallel Seq Scan

* Jim Nasby (Jim(dot)Nasby(at)BlueTreble(dot)com) wrote:
> On 1/5/15, 9:21 AM, Stephen Frost wrote:
> >* Robert Haas (robertmhaas(at)gmail(dot)com) wrote:
> >>I think it's right to view this in the same way we view work_mem. We
> >>plan on the assumption that an amount of memory equal to work_mem will
> >>be available at execution time, without actually reserving it.
> >
> >Agreed- this seems like a good approach for how to address this. We
> >should still be able to end up with plans which use less than the max
> >possible parallel workers though, as I pointed out somewhere up-thread.
> >This is also similar to work_mem- we certainly have plans which don't
> >expect to use all of work_mem and others that expect to use all of it
> >(per node, of course).
>
> I agree, but we should try and warn the user if they set parallel_seqscan_degree close to max_worker_processes, or at least give some indication of what's going on. This is something you could end up beating your head on wondering why it's not working.
>
> Perhaps we could have EXPLAIN throw a warning if a plan is likely to get less than parallel_seqscan_degree number of workers.

Yeah, if we come up with a plan for X workers and end up not being able
to spawn that many then I could see that being worth a warning or notice
or something. Not sure what EXPLAIN has to do anything with it..

Thanks,

Stephen