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From Ali Safdar Kureishy <>
Subject Re: Storage requirements for intermediate (map-side-output) data during Hive joins
Date Mon, 07 May 2012 07:38:15 GMT
Please ignore my question below. I made a mistake with my calculation. The
map-side joins do not perform a cross-product of the data. They just emit
the data using the join-key as the row key.


On Mon, May 7, 2012 at 12:31 AM, Ali Safdar Kureishy <> wrote:

> Hi,
> I'm setting up a Hadoop cluster and would like to understand how much disk
> space I should expect to need with joins.
> Let's assume that I have 2 tables, each of about 500 GB. Since the tables
> are large, these will all be reduce-side joins. As far as I know about such
> joins, the data generated is a cross product of the size of the two tables.
> Am I wrong?
> In other words, for a reduce-side join in Hive involving 2 such tables,
> would I need to accommodate for 500 GB * 500 GB = *250000 *GB of *
> intermediate* (map-side output) data before the reducer(s) kick-in in my
> cluster? Or am I missing something? That seems rediculously high, so I hope
> I'm mistaken.
> But if the above IS accurate, what are the ways to reduce this consumption
> for the same kind of join in Hive?
> Thanks,
> Safdar

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