Re: Parallel Seq Scan

On 12 November 2015 at 15:23, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Wed, Nov 11, 2015 at 11:29 PM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
> wrote:
>>
>> Hi
>>
>> I have a first query
>>
>> I looked on EXPLAIN ANALYZE output and the numbers of filtered rows are
>> differen
>>
>
> Thanks for the report. The reason for this problem is that instrumentation
> information from workers is getting aggregated multiple times. In
> ExecShutdownGatherWorkers(), we call ExecParallelFinish where it
> will wait for workers to finish and then accumulate stats from workers.
> Now ExecShutdownGatherWorkers() could be called multiple times
> (once we read all tuples from workers, at end of node) and it should be
> ensured that repeated calls should not try to redo the work done by first
> call.
> The same is ensured for tuplequeues, but not for parallel executor info.
> I think we can safely assume that we need to call ExecParallelFinish() only
> when there are workers started by the Gathers node, so on those lines
> attached patch should fix the problem.

That fixes the count issue for me, although not the number of buffers
hit, or the actual time taken.

Thom