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From "Hostettler, Steve" <Steve.Hostett...@wolterskluwer.com>
Subject RE: New SQL execution engine
Date Mon, 18 Nov 2019 10:13:08 GMT
Hi Roman,

Thanks a lot for the answer (and the pull request). As I said initially, I was under the impression
that the reason was the lack of affinity.
I understand the reason and the current design and I think we all agreed that this is not
optimal and that it should be reworked in the new design. Especially the sort of silent behavior.
That being said, more than a warning : having joins in // inter partitions would be very helpful
but I understand that it is not straightforward.

As always you guys are very reactive and helpful. Keep up the great work. Appreciate it.

-----Original Message-----
From: Roman Kondakov <kondakov87@mail.ru.INVALID> 
Sent: Monday, November 18, 2019 11:04 AM
To: dev@ignite.apache.org
Subject: Re: New SQL execution engine

Hi, Steve

This behavior is actually not a bug, but this is not obvious. I'll try to explain.

When query parallelism = N is turned on, it means that each cache is divided into N parts
from the SQL point of view. Every SQL query is executed independently over each particular
part, and then results are merged together during the reducer step.

This is absolutely identical to the distributed query execution, where instead of a single
node with query parallelism = N, we have N nodes with query parallelism = 1. SQL query is
executed over each partition of data on all nodes and then results are merged on reducer.

As we can see, query parallelism is equivalent to the distributed query execution. When we
do joins over distributed tables, we need to think about the collocation of data [1]. If data
is not collocated, we get a wrong result. This happens silently, which is not good, IMO.

I reworked your example a bit in order to impose collocation on the joining key and now join
returns correct result [2].

Current approach in configuration and query execution looks very uncomfortable and should
be completely redesigned in the new engine.

[1] https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fapacheignite-sql.readme.io%2Fdocs%2Fdistributed-joins&amp;data=02%7C01%7CSteve.Hostettler%40wolterskluwer.com%7C68a93ad417fc4e70ed1808d76c0e9f53%7C8ac76c91e7f141ffa89c3553b2da2c17%7C0%7C0%7C637096682368420072&amp;sdata=82bDWI1PHUOzNz95A5F%2Flyiqlrb9aQ2vadxhE%2FK47LM%3D&amp;reserved=0

[2] https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fhostettler%2FigniteParallelQueries%2Fpull%2F1&amp;data=02%7C01%7CSteve.Hostettler%40wolterskluwer.com%7C68a93ad417fc4e70ed1808d76c0e9f53%7C8ac76c91e7f141ffa89c3553b2da2c17%7C0%7C0%7C637096682368420072&amp;sdata=QCvNEKqGGyZYOXQbF0sG0DUCzYJCnKoWleFTMtngcsc%3D&amp;reserved=0

Kind Regards
Roman Kondakov

On 16.11.2019 12:50, steve.hostettler@gmail.com wrote:
> Actually I am now wondering whether this is not just a bug and that I 
> should record it as such. As the behavior is different with and 
> without the parallelism and there is no warning during execution or in the api.
> Any thought?
> --
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