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Hive, mail # user - non-equality joins


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Re: non-equality joins
Keith Wiley 2012-03-13, 16:49
https://cwiki.apache.org/Hive/languagemanual-joins.html

"Hive does not support join conditions that are not equality conditions as it is very difficult to express such conditions as a map/reduce job."

I admit, that isn't a very detailed answer, but it gives some indication of the reason for the discrepancy between Hive and other databases.  Hive fundamentally operates on Hadoop, namely on MapReduce (we all know this, I'm just reiterating the train of thought).  The problem is that certain algorithms are exceedingly difficult to wedge into the MapReduce framework.

That is as detailed as my personal insight can get.  I've done a lot of MapReduce programming in Hadoop but I'm not a database expert and I don't really understand the steps involved in various kinds of table-joins, so I don't understand the particular ways in which certain database operations do or do not fit into MapReduce...but presumably nonequality joins (whatever those are :-D ) are particularly difficult to MapReduceify.

Cheers!

On Mar 13, 2012, at 09:17 , mahsa mofidpoor wrote:

> Hello,
>
> Is there a reason behind not implementing non-equality joins in Hive? In other words, is there any usage for theta-join, if implemented?
>
> Thank you in advance for your response,
> Mahsa
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Keith Wiley     [EMAIL PROTECTED]     keithwiley.com    music.keithwiley.com

"It's a fine line between meticulous and obsessive-compulsive and a slippery
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                                           --  Keith Wiley
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