On Fri, Apr 29, 2016 at 7:33 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Amit Kapila <amit.kapil...@gmail.com> writes:
> >> On Thu, Apr 28, 2016 at 10:06 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> >>> I'd be inclined to think that it's silly to build GatherPaths in
> >>> of having finalized the list of partial paths for a rel.
> > What's happening here is that to form joinrel, we need to call
> > add_paths_to_joinrel() with both outer and inner relation twice, once
> > rel1 as outer relation and rel1 as inner relation and vice versa.  So
> > the second call to add_paths_to_joinrel() can replace a partial path
> > is being referenced by GatherPath generated in first call.  I think we
> > should generate gather paths for join rel after both the calls
> > to add_paths_to_joinrel() aka in make_join_rel().  Attached patch on
> > lines fixes the problem for me.
> make_join_rel is certainly not far enough down the call stack to solve
> this problem.  It can, and typically will, be invoked multiple times
> for the same target join relation.
> One possible answer is to do it in standard_join_search, just before
> the set_cheapest call for each join relation.

Yes, that makes sense to me.

>  You'd need to account
> for the issue in GEQO search as well.

How about doing it in merge_clump() before calling set_cheapest()?

Yet, another idea could be to create a copy of partial path before passing
it to create_gather_path()?

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

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