> But why disable mapjoin has better performance when we don't use cast to
>string(user always lazy)?
>
> Join key values comparison in in reduce stage is more quickly?
The HashMap<DoubleWritable, RowContainer> is slower than the full-sort +
sorted-merge-join.
It shouldn't be, but it hits the worst-case performance for the Hashmap
impl because of a bug in DoubleWritable in Hadoop.
The effect is somewhat the same as
public int hashCode() {
return 1;
}
Read the comments on - https://issues.apache.org/jira/browse/HADOOP-12217
Cheers,
Gopal
