Re: Parallel Seq Scan

On 9 April 2015 at 00:12, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:

> On Wed, Apr 8, 2015 at 3:30 PM, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> >
> > On 8 April 2015 at 15:46, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >>
> >> I think there is always a chance that resources (like parallel-workers)
> >> won't be available at run-time even if we decide about them at
> >> executor-start phase unless we block it for that node's usage and OTOH
> >> if we block it (by allocating) those resources during executor-start
> phase
> >> then we might end up blocking it too early or may be they won't even get
> >> used if we decide not to execute that node. On that basis, it seems to
> >> me current strategy is not bad where we decide during planning time and
> >> later during execution time if not all resources (particularly
> parallel-workers)
> >> are not available, then we use only the available one's to execute the
> plan.
> >> Going forward, I think we can improve the same if we decide not to
> shutdown
> >> parallel workers till postmaster shutdown once they are started and
> >> then just allocate them during executor-start phase.
> >>
> >>
> >
> > Yeah, but what about when workers are not available in cases when the
> plan was only a win because the planner thought there would be lots of
> workers... There could have been a more optimal serial plan already thrown
> out by the planner which is no longer available to the executor.
> >
>
> That could also happen even if we decide in executor-start phase.
>

Yes this is true, but if we already have the most optimal serial plan, then
there's no issue.

> I agree that there is a chance of loss incase appropriate resources
> are not available during execution, but same is true for work_mem
> as well for a non-parallel plan. I think we need some advanced way
> to handle the case when resources are not available during execution
> by either re-planing the statement or by some other way, but that can
> also be done separately.
>
>
There was some talk of re-planning queries over on the Removing INNER JOINs
thread:
http://www.postgresql.org/message-id/CA+TgmoaHi8tq7haZCf46O_NUHT8w=P0Z_N59DC0yOjfMucS9bg@mail.gmail.com

Regards

David Rowley