Re: Parallel Seq Scan

On Wed, Jan 21, 2015 at 12:47 PM, Amit Langote <
Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>
> On 20-01-2015 PM 11:29, Amit Kapila wrote:
> > Note - I have yet to handle the new node types introduced at some
> > of the places and need to verify prepared queries and some other
> > things, however I think it will be good if I can get some feedback
> > at current stage.
> >
>
> I got an assertion failure:
>
> In src/backend/executor/execTuples.c: ExecStoreTuple()
>
> /* passing shouldFree=true for a tuple on a disk page is not sane */
> Assert(BufferIsValid(buffer) ? (!shouldFree) : true);
>

Good Catch!
The reason is that while master backend is scanning from a heap
page, if it finds another tuple/tuples's from shared memory message
queue it will process those tuples first and in such a scenario, the scan
descriptor will still have reference to buffer which it is using from
scanning
from heap. Your proposed fix will work.

> After fixing this, the assertion failure seems to be gone though I
> observed the blocked (CPU maxed out) state as reported elsewhere by Thom
> Brown.
>

Does it happen only when parallel_seqscan_degree > max_worker_processes?

Thanks for checking the patch.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com