Consider the query like:
( select idOne, idTwo, value FROM
smallTableOne on (bigTable.idOne = smallTableOne.idOne)
smallTableTwo on (firstjoin.idTwo = smallTableTwo.idTwo);
where smallTableOne and smallTableTwo are smaller than hive.auto.convert.join.noconditionaltask.size and
hive.auto.convert.join.noconditionaltask is set to true.
The joins are collapsed into mapjoins, and it leads to a map-only job
(for the map-joins) followed by a map-reduce job (for the group by).
Ideally, the map-only job should be merged with the following map-reduce job.