Re: [HACKERS] Parallel Hash take II

On Thu, Sep 14, 2017 at 11:57 AM, Thomas Munro
<thomas.munro@enterprisedb.com> wrote:
> On Thu, Sep 14, 2017 at 12:51 AM, Prabhat Sahu
> <prabhat.sahu@enterprisedb.com> wrote:
>> Setting with lower "shared_buffers" and "work_mem" as below,  query getting crash but able to see explain plan.
>
> Thanks Prabhat.  A small thinko in the batch reset code means that it
> sometimes thinks the shared skew hash table is present and tries to
> probe it after batch 1.  I have a fix for that and I will post a new
> patch set just as soon as I have a good regression test figured out.

Fixed in the attached version, by adding a missing
"hashtable->shared->num_skew_buckets = 0;" to ExecHashFreeSkewTable().
I did some incidental tidying of the regression tests, but didn't
manage to find a version of your example small enough to put in a
regression tests.  I also discovered some other things:

1.  Multi-batch Parallel Hash Join could occasionally produce a
resowner warning about a leaked temporary File associated with
SharedTupleStore objects.  Fixed by making sure we call routines that
close all files handles in ExecHashTableDetach().

2.  Since last time I tested, a lot fewer TPCH queries choose a
Parallel Hash plan.  Not sure why yet.  Possibly because Gather Merge
and other things got better.  Will investigate.

3.  Gather Merge and Parallel Hash Join may have a deadlock problem.
Since Gather Merge needs to block waiting for tuples, but workers wait
for all participants (including the leader) to reach barriers.  TPCH
Q18 (with a certain set of indexes and settings, YMMV) has Gather
Merge over Sort over Parallel Hash Join, and although it usually runs
successfully I have observed one deadlock.  Ouch.  This seems to be a
more fundamental problem than the blocked TupleQueue scenario.  Not
sure what to do about that.

-- 
Thomas Munro
http://www.enterprisedb.com

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