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.
Thanks, Safdar On Mon, May 7, 2012 at 12:31 AM, Ali Safdar Kureishy < safdar.kurei...@gmail.com> 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 >
