Re: crashes due to setting max_parallel_workers=0

Thanks Tomas for reporting issue and Thanks David for working
on this.

I can see the problem in GatherMerge, specially when nworkers_launched
is zero. I will look into this issue and will post a fix for the same.

Also another point which I think we should fix is, when someone set
max_parallel_workers = 0, we should also set
the max_parallel_workers_per_gather
to zero. So that way it we can avoid generating the gather path with
max_parallel_worker = 0.

Thanks,


On Sat, Mar 25, 2017 at 2:21 PM, David Rowley <david.rowley@2ndquadrant.com>
wrote:

> On 25 March 2017 at 13:10, Tomas Vondra <tomas.vondra@2ndquadrant.com>
> wrote:
> > while working on a patch I ran into some crashes that seem to be caused
> by
> > inconsistent handling of max_parallel_workers - queries still seem to be
> > planned with parallel plans enabled, but then crash at execution.
> >
> > The attached script reproduces the issue on a simple query, causing
> crashed
> > in GatherMerge, but I assume the issue is more general.
>
> I had a look at this and found a bunch of off by 1 errors in the code.
>
> I've attached a couple of patches, one is the minimal fix, and one
> more complete one.
>
> In the more complete one I ended up ditching the
> GatherMergeState->nreaders altogether. It was always the same as
> nworkers_launched anyway, so I saw no point in it.
> Here I've attempted to make the code a bit more understandable, to
> prevent further confusion about how many elements are in each array.
> Probably there's more that can be done here. I see GatherState has
> nreaders too, but I've not looked into the detail of if it suffers
> from the same weirdness of nreaders always matching nworkers_launched.
>
>
> --
>  David Rowley                   http://www.2ndQuadrant.com/
>  PostgreSQL Development, 24x7 Support, Training & Services
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-- 
Rushabh Lathia