Re: Gather Merge

On Fri, Mar 10, 2017 at 4:09 PM, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>
wrote:

> On Fri, Mar 10, 2017 at 3:04 PM, Rushabh Lathia
> <rushabh(dot)lathia(at)gmail(dot)com> wrote:
> >
> >
> > On Fri, Mar 10, 2017 at 2:42 PM, Andreas Joseph Krogh <
> andreas(at)visena(dot)com>
> > wrote:
> >>
> >> På fredag 10. mars 2017 kl. 10:09:22, skrev Rushabh Lathia
> >> <rushabh(dot)lathia(at)gmail(dot)com>:
> >>
> >>
> >>
> >> On Fri, Mar 10, 2017 at 2:33 PM, Andreas Joseph Krogh <
> andreas(at)visena(dot)com>
> >> wrote:
> >>>
> >>> [...]
> >>> The execution-plan seems (unsurprisingly) to depend on
> data-distribution,
> >>> so is there a way I can force a GatherMerge?
> >>
> >>
> >> Not directly. GatherMerge cost is mainly depend on parallel_setup_cost,
> >> parallel_tuple_cost and cpu_operator_cost. May be you can force this
> >> by setting this cost low enough. Or another way to force is by disable
> the
> >> other plans.
> >>
> >> What plan you are getting now? You not seeing the below error ?
> >>
> >> ERROR: GatherMerge child's targetlist doesn't match GatherMerge
> >>
> >>
> >> I'm seeing the same error, it's just that for reproducing it I'd rather
> >> not copy my whole dataset.
> >
> >
> > Can you share me a schema information, I will try to reproduce at my
> side?
> I'm able to reproduce the error. I've attached the dump file and a
> script to reproduce it.
>
> The following query executes successfully.
> postgres=# explain select t1.* from t1 JOIN t2 ON t1.k=t2.k where
> t1.i=1 order by t1.j desc;
> QUERY PLAN
> ------------------------------------------------------------
> -------------------------------------------
> Gather Merge (cost=0.58..243.02 rows=943 width=12)
> Workers Planned: 1
> -> Nested Loop (cost=0.57..235.94 rows=555 width=12)
> -> Parallel Index Scan Backward using idx_t1_i_j on t1
> (cost=0.29..14.33 rows=603 width=12)
> Index Cond: (i = 1)
> -> Index Only Scan using idx_t2_k on t2 (cost=0.29..0.34
> rows=3 width=4)
> Index Cond: (k = t1.k)
> (7 rows)
>
> Whereas, If columns from t2 is projected, it throws the same error.
> postgres=# explain select t2.* from t1 JOIN t2 ON t1.k=t2.k where
> t1.i=1 order by t1.j desc;
> ERROR: GatherMerge child's targetlist doesn't match GatherMerge
>
>
Thanks Kuntal.

I am able to reproduce the issue with the shared script. I will look into
this now.

>
>
> --
> Thanks & Regards,
> Kuntal Ghosh
> EnterpriseDB: http://www.enterprisedb.com
>

--
Rushabh Lathia