Re: Parallel Seq Scan

On 1/5/15, 9:21 AM, Stephen Frost wrote:
> * Robert Haas (robertmhaas@gmail.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
leastgive some indication of what's going on. This is something you could end up beating your head on wondering why
it'snot 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
Data in Trouble? Get it in Treble! http://BlueTreble.com