Resolution: Not A Problem
Affects Version/s: None
Fix Version/s: None
Component/s: Query Processor
explain SELECT x.key, count(1) FROM src1 x JOIN src y ON (x.key = y.key) group by x.key;
Stage-1 is a root stage
Stage-2 depends on stages: Stage-1
Stage-0 is a root stage
The above query issues 2 map-reduce jobs.
The first MR job performs the join, whereas the second MR performs the group by.
Since the data is already sorted, the group by can be performed in the reducer of the join itself.