org.apache.crunch.lib.join

Class MapsideJoin

java.lang.Object

org.apache.crunch.lib.join.MapsideJoin

public class MapsideJoin
extends Object

Utility for doing map side joins on a common key between two PTables.

A map side join is an optimized join which doesn't use a reducer; instead, the right side of the join is loaded into memory and the join is performed in a mapper. This style of join has the important implication that the output of the join is not sorted, which is the case with a conventional (reducer-based) join.

Note:This utility is only supported when running with a MRPipeline as the pipeline.

Constructor Summary

Constructors

Constructor and Description
MapsideJoin()

Method Summary

Methods

Modifier and Type	Method and Description
static <K,U,V> PTable<K,Pair<U,V>>	join(PTable<K,U> left, PTable<K,V> right) Join two tables using a map side join.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

MapsideJoin

public MapsideJoin()

Method Detail

join

public static <K,U,V> PTable<K,Pair<U,V>> join(PTable<K,U> left,
                               PTable<K,V> right)

Join two tables using a map side join. The right-side table will be loaded fully in memory, so this method should only be used if the right side table's contents can fit in the memory allocated to mappers. The join performed by this method is an inner join.

Parameters:

left - The left-side table of the join

right - The right-side table of the join, whose contents will be fully read into memory

Returns:

A table keyed on the join key, containing pairs of joined values