Re: Gather Merge

On Thu, Mar 9, 2017 at 9:42 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Thu, Mar 9, 2017 at 8:21 AM, Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com>
> wrote:
> > Thanks Robert for committing this.
> >
> > My colleague Neha Sharma found one regression with the patch. I was about
> > to send this mail and noticed that you committed the patch.
>
> Oops. Bad timing.
>
> > postgres=# explain select aid from pgbench_accounts where aid % 25= 0
> group
> > by aid;
> > ERROR: ORDER/GROUP BY expression not found in targetlist
>
> I think your fix for this looks right, although I would write it this way:
>
> - gm_plan->plan.targetlist = subplan->targetlist;
> + gm_plan->plan.targetlist = build_path_tlist(root, &best_path->path);
>
> The second part of your fix looks wrong. I think you want this:
>
> create_gather_merge_path(root,
> grouped_rel,
> subpath,
> - NULL,
> + partial_grouping_target,
> root->group_pathkeys,
> NULL,
> &total_groups);
>
> That will match the create_gather_path case.
>
>
Right, I did that change and perform the test with the fix and I don't
see any regression now.

> This test case is still failing for me even with those fixes:
>
> rhaas=# select aid+1 from pgbench_accounts group by aid+1;
> ERROR: could not find pathkey item to sort
>
>
I don't see this failure with the patch. Even I forced the gather merge
in the above query and that just working fine.

Attaching patch, with the discussed changes.

Thanks,
Rushabh Lathia