On Thu, Jun 16, 2016 at 8:00 AM, Amit Kapila <amit.kapil...@gmail.com> wrote:
>> 1. The case originally reported by Thomas Munro still fails.  To fix
>> that, we probably need to apply scanjoin_target to each partial path.
>> But we can only do that if it's parallel-safe.  It seems like what we
>> want is something like this: (1) During scan/join planning, somehow
>> skip calling generate_gather_paths for the topmost scan/join rel as we
>> do to all the others.  (2) If scanjoin_target is not parallel-safe,
>> build a path for the scan/join phase that applies a Gather node to the
>> cheapest path and does projection at the Gather node.  Then forget all
>> the partial paths so we can't do any bogus upper planning.  (3) If
>> scanjoin_target is parallel-safe, replace the list of partial paths
>> for the topmost scan/join rel with a new list where scanjoin_target
>> has been applied to each one.  I haven't tested this so I might be
>> totally off-base about what's actually required here...
>
> I think we can achieve it just by doing something like what you have
> mentioned in (2) and (3).  I am not sure if there is a need to skip
> generation of gather paths for top scan/join node.  Please see the patch
> attached.  I have just done some minimal testing to ensure that problem
> reported by Thomas Munro in this thread is fixed and verified that the fix
> is sane for problems [1][2] reported by sqlsmith. If you think this is on
> right lines, I can try to do more verification and try to add tests.

You can't do it this way because of the issue Tom discussed here:

https://www.postgresql.org/message-id/16421.1465828...@sss.pgh.pa.us

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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