Hive 执行计划

执行语句

hive> explain select s.id, s.name from student s left outer join student_tmp st on s.name = st.name;

结果，红色字体为我添加的注释

hive> explain select s.id, s.name from student s left outer join student_tmp st on s.name = st.name;
OK
ABSTRACT SYNTAX TREE:
  (TOK_QUERY (TOK_FROM (TOK_LEFTOUTERJOIN (TOK_TABREF (TOK_TABNAME student) s) (TOK_TABREF (TOK_TABNAME student_tmp) st) (= (. (TOK_TABLE_OR_COL s) name) (. (TOK_TABLE_OR_COL st) name)))) (TOK_INSERT (TOK_DESTINATION (TOK_DIR TOK_TMP_FILE)) (TOK_SELECT (TOK_SELEXPR (. (TOK_TABLE_OR_COL s) id)) (TOK_SELEXPR (. (TOK_TABLE_OR_COL s) name)))))

STAGE DEPENDENCIES: “这个sql将被分成两个阶段执行。基本上每个阶段会对应一个mapreduce job，Stage-0除外。因为Stage-0只是fetch结果集，不需要mapreduce job”
  Stage- is a root stage
  Stage- is a root stage

STAGE PLANS:
  Stage: Stage-
    Map Reduce
      Alias -> Map Operator Tree: “map job开始”
        s
          TableScan
            alias: s “扫描表student”
            Reduce Output Operator “这里描述map的输出，也就是reduce的输入。比如key，partition，sort等信息。”
              key expressions: “reduce job的key”
                    expr: name
                    type: string
              sort order: + “这里表示按一个字段排序，如果是按两个字段排序，那么就会有两个+(++),更多以此类推”
              Map-reduce partition columns: “partition的信息，由此也可以看出hive在join的时候会以join on后的列作为partition的列，以保证具有相同此列的值的行被分到同一个reduce中去”
                    expr: name
                    type: string
              tag: 0 “用于标示这个扫描的结果，后面的join会用到它”
              value expressions: “表示select 后面的列”
                    expr: id
                    type: int
                    expr: name
                    type: string
        st
          TableScan “开始扫描第二张表，和上面的一样”
            alias: st
            Reduce Output Operator
              key expressions:
                    expr: name
                    type: string
              sort order: +
              Map-reduce partition columns:
                    expr: name
                    type: string
              tag:
      Reduce Operator Tree: “reduce job开始”
        Join Operator
          condition map:
               Left Outer Join0 to 1 “tag 0 out join tag 1”
          condition expressions: “这里也是描述select 后的列，和join没有关系。这里我们的select后的列是 s.id 和 s.name, 所以0后面有两个字段， 1后面没有”
             {VALUE._col0} {VALUE._col2} 

          handleSkewJoin: false
          outputColumnNames: _col0, _col2
          Select Operator
            expressions:
                  expr: _col0
                  type: int
                  expr: _col2
                  type: string
            outputColumnNames: _col0, _col1
            File Output Operator
              compressed: false
              GlobalTableId:
              table:
                  input format: org.apache.hadoop.mapred.TextInputFormat
                  output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat

  Stage: Stage-
    Fetch Operator
      limit: -

Time taken: 0.216 seconds