Re: Gather Merge

Error coming from create_gather_merge_plan() from below condition:

if (memcmp(sortColIdx, gm_plan->sortColIdx,
numsortkeys * sizeof(AttrNumber)) != 0)
elog(ERROR, "GatherMerge child's targetlist doesn't match
GatherMerge");

Above condition checks the sort column numbers explicitly, to ensure the
tlists
really do match up. This been copied from the create_merge_append_plan().
Now
this make sense as for MergeAppend as its not projection capable plan (see
is_projection_capable_plan()). But like Gather, GatherMerge is the
projection
capable and its target list can be different from the subplan, so I don't
think this
condition make sense for the GatherMerge.

Here is the some one the debugging info, through which I was able to reach
to this conclusion:

- targetlist for the GatherMerge and subpath is same during
create_gather_merge_path().

- targetlist for the GatherMerge is getting changed into
create_gather_merge_plan().

postgres=# explain (analyze, verbose) select t2.j from t1 JOIN t2 ON
t1.k=t2.k where t1.i=1 order by t1.j desc;
NOTICE: path parthtarget: {PATHTARGET :exprs ({VAR :varno 2 :varattno 2
:vartype 23 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnoold 2
:varoattno 2 :location 34} {VAR :varno 1 :varattno 2 :vartype 23 :vartypmod
-1 :varcollid 0 :varlevelsup 0 :varnoold 1 :varoattno 2 :location 90})
:sortgrouprefs 0 1 :cost.startup 0.00 :cost.per_tuple 0.00 :width 8}

NOTICE: subpath parthtarget: {PATHTARGET :exprs ({VAR :varno 1 :varattno 2
:vartype 23 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnoold 1
:varoattno 2 :location 90} {VAR :varno 2 :varattno 2 :vartype 23 :vartypmod
-1 :varcollid 0 :varlevelsup 0 :varnoold 2 :varoattno 2 :location 34})
:cost.startup 0.00 :cost.per_tuple 0.00 :width 8}

- Attached memory watch point and found that target list for GatherMerge is
getting
changed into groupping_planner() -> apply_projection_to_path().

PFA patch to fix this issue.

On Fri, Mar 10, 2017 at 4:12 PM, Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com>
wrote:

>
>
> 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
>

--
Rushabh Lathia