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From: Dmitriy Setrakyan <dsetrakyan@apache.org>
Date: Wed, 31 May 2017 11:26:38 -0700
Message-ID: <CA+0=VoXGxY7Wdp9JoZm=czcEdN7qCr4Ra+q3ZEyOn5DNUGVxWA@mail.gmail.com>
Subject: nested SQL sub-queries with non-collocated joins
To: dev@ignite.apache.org
Cc: Sergi Vladykin <sergi.vladykin@gmail.com>
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archived-at: Wed, 31 May 2017 18:27:21 -0000

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Igniters (specifically Sergi),

It has come to my attention today that nested sub-select statements, when
used in combination with non-collocated joins do not work properly in
Ignite.

So a query like this, where A, B, and C are all stored in Partitioned
caches and are **not** collocated at all, will not work.


> *select * from A, B where a.id <http://a.id> = b.a_id and b.somefield in
> (select somefield from C where c.zipcode = ?)*


The main reason it is not supported right now is because, in the absence of
collocation, such query may create N^N complexity and it was decided that
it is best not supporting it at all.

However, I am not sure why N^N complexity is required. Why not support it
as follows?

   1. execute the nested subquery and store the result in a temporary
   Replicated table.
   2. execute the original query and use the temporary Replicated table
   instead of the sub-query.

Sergi, given that you are the author of the code, can you provide some
insight here?

Thanks,
D.

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