Re: [HACKERS] Parallel Append implementation

On Wed, Aug 30, 2017 at 5:32 PM, Amit Khandekar <amitdkhan.pg@gmail.com> wrote:
> Hi Rafia,
>
> On 17 August 2017 at 14:12, Amit Khandekar <amitdkhan.pg@gmail.com> wrote:
>> But for all of the cases here, partial
>> subplans seem possible, and so even on HEAD it executed Partial
>> Append. So between a Parallel Append having partial subplans and a
>> Partial Append having partial subplans , the cost difference would not
>> be significant. Even if we assume that Parallel Append was chosen
>> because its cost turned out to be a bit cheaper, the actual
>> performance gain seems quite large as compared to the expected cost
>> difference. So it might be even possible that the performance gain
>> might be due to some other reasons. I will investigate this, and the
>> other queries.
>>
>
> I ran all the queries that were showing performance benefits in your
> run. But for me, the ParallelAppend benefits are shown only for plans
> that use Partition-Wise-Join.
>
> For all the queries that use only PA plans but not PWJ plans, I got
> the exact same plan for HEAD as for PA+PWJ patch, except that for the
> later, the Append is a ParallelAppend. Whereas, for you, the plans
> have join-order changed.
>
> Regarding actual costs; consequtively, for me the actual-cost are more
> or less the same for HEAD and PA+PWJ. Whereas, for your runs, you have
> quite different costs naturally because the plans themselves are
> different on head versus PA+PWJ.
>
> My PA+PWJ plan outputs (and actual costs) match exactly what you get
> with PA+PWJ patch. But like I said, I get the same join order and same
> plans (and actual costs) for HEAD as well (except
> ParallelAppend=>Append).
>
> May be, if you have the latest HEAD code with your setup, you can
> yourself check some of the queries again to see if they are still
> seeing higher costs as compared to PA ? I suspect that some changes in
> latest code might be causing this discrepancy; because when I tested
> some of the explains with a HEAD-branch server running with your
> database, I got results matching PA figures.
>
> Attached is my explain-analyze outputs.
>

Now, when I compare your results with the ones I posted I could see
one major difference between them -- selectivity estimation errors.
In the results I posted, e.g. Q3, on head it gives following

->  Finalize GroupAggregate  (cost=41131358.89..101076015.45
rows=455492628 width=44) (actual time=126436.642..129247.972
rows=226765 loops=1)              Group Key: lineitem_001.l_orderkey,
orders_001.o_orderdate, orders_001.o_shippriority              ->  Gather Merge  (cost=41131358.89..90637642.73
rows=379577190 width=44) (actual time=126436.602..127791.768
rows=235461 loops=1)                    Workers Planned: 2                    Workers Launched: 2

and in your results it is,
->  Finalize GroupAggregate  (cost=4940619.86..6652725.07
rows=13009521 width=44) (actual time=89573.830..91956.956 rows=226460
loops=1)              Group Key: lineitem_001.l_orderkey,
orders_001.o_orderdate, orders_001.o_shippriority              ->  Gather Merge  (cost=4940619.86..6354590.21
rows=10841268 width=44) (actual time=89573.752..90747.393 rows=235465
loops=1)                    Workers Planned: 2                    Workers Launched: 2

However, for the results with the patch/es this is not the case,

in my results, with patch,
->  Finalize GroupAggregate  (cost=4933450.21..6631111.01
rows=12899766 width=44) (actual time=87250.039..90593.716 rows=226765
loops=1)              Group Key: lineitem_001.l_orderkey,
orders_001.o_orderdate, orders_001.o_shippriority              ->  Gather Merge  (cost=4933450.21..6335491.38
rows=10749804 width=44) (actual time=87250.020..89125.279 rows=227291
loops=1)                    Workers Planned: 2                    Workers Launched: 2

I think this explains the reason for drastic different in the plan
choices and thus the performance for both the cases.

Since I was using same database for the cases, I don't have much
reasons for such difference in selectivity estimation for these
queries. The only thing might be a missing vacuum analyse, but since I
checked it a couple of times I am not sure if even that could be the
reason. Additionally, it is not the case for all the queries, like in
Q10 and Q21, the estimates are similar.

However, on a fresh database the selectivity-estimates and plans as
reported by you and with the patched version I posted seems to be the
correct one. I'll see if I may check performance of these queries once
again to verify these.

-- 
Regards,
Rafia Sabih
EnterpriseDB: http://www.enterprisedb.com/


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