On Wed, Apr 8, 2015 at 3:30 PM, David Rowley <dgrowleyml@gmail.com> wrote: > > On 8 April 2015 at 15:46, Amit Kapila <amit.kapila16@gmail.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: