Re: Parallel Seq Scan

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.
--
Jim Nasby, Data Architect, Blue Treble Consulting
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