奇怪的Hive输出

Question

这是Hive SQL查询的输出，想知道Map1和Map3在这里是什么意思？对于Reducer 2，“2”表示？

这是Hive SQL查询和输出，

select student_id, student_name
from
        (select student_id, student_name
        from students
        where grade='G1' and class in (3427, 34280)) T1
        inner join
        (select student_id from activities where is_active=true ) T2
        on T1.student_id=T2.student_id
limit 100;

Map 1: 21/359   Map 3: 1/1  Reducer 2: 0/59
Map 1: 22/359   Map 3: 1/1  Reducer 2: 0/59
Map 1: 23/359   Map 3: 1/1  Reducer 2: 0/59
Map 1: 25/359   Map 3: 1/1  Reducer 2: 0/59
Map 1: 26/359   Map 3: 1/1  Reducer 2: 0/59
Map 1: 27/359   Map 3: 1/1  Reducer 2: 0/59

提前谢谢， 林