Hive Join Explain

对于一下两种join方式，执行计划结果是一致的，都是只有一个MR

explain select * from tb1 t1
LEFT JOIN tb2 t2 ON t1.name = t2.name
LEFT JOIN tb3 t3 ON t2.name = t3.name;

explain select * from tb1 t1
LEFT JOIN tb2 t2 ON t1.name = t2.name
LEFT JOIN tb3 t3 ON t1.name = t3.name;

STAGE DEPENDENCIES:
  Stage-5 is a root stage
  Stage-4 depends on stages: Stage-5
  Stage-0 depends on stages: Stage-4
STAGE PLANS:
  Stage: Stage-5
    Map Reduce Local Work
      Alias -> Map Local Tables:
        t2 
          Fetch Operator
            limit: -1
        t3 
          Fetch Operator
            limit: -1
      Alias -> Map Local Operator Tree:
        t2 
          TableScan
            alias: t2
            Statistics: Num rows: 1 Data size: 0 Basic stats: PARTIAL Column stats: NONE
            HashTable Sink Operator
              keys:
                0 name (type: string)
                1 name (type: string)
                2 name (type: string)
        t3 
          TableScan
            alias: t3
            Statistics: Num rows: 1 Data size: 0 Basic stats: PARTIAL Column stats: NONE
            HashTable Sink Operator
              keys:
                0 name (type: string)
                1 name (type: string)
                2 name (type: string)
  Stage: Stage-4
    Map Reduce
      Map Operator Tree:
          TableScan
            alias: t1
            Statistics: Num rows: 1 Data size: 0 Basic stats: PARTIAL Column stats: NONE
            Map Join Operator
              condition map:
                   Left Outer Join0 to 1
                   Left Outer Join1 to 2
              keys:
                0 name (type: string)
                1 name (type: string)
                2 name (type: string)
              outputColumnNames: _col0, _col1, _col5, _col6, _col10, _col11
              Statistics: Num rows: 2 Data size: 0 Basic stats: PARTIAL Column stats: NONE
              Select Operator
                expressions: _col0 (type: int), _col1 (type: string), _col5 (type: int), _col6 (type: string), _col10 (type: int), _col11 (type: string)
                outputColumnNames: _col0, _col1, _col2, _col3, _col4, _col5
                Statistics: Num rows: 2 Data size: 0 Basic stats: PARTIAL Column stats: NONE
                File Output Operator
                  compressed: false
                  Statistics: Num rows: 2 Data size: 0 Basic stats: PARTIAL Column stats: NONE
                  table:
                      input format: org.apache.hadoop.mapred.SequenceFileInputFormat
                      output format: org.apache.hadoop.hive.ql.io.HiveSequenceFileOutputFormat
                      serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe
      Local Work:
        Map Reduce Local Work
  Stage: Stage-0
    Fetch Operator
      limit: -1
      Processor Tree:
        ListSink