select key from src group by key;の実行計画

gistfile1.txt

PREHOOK: query: explain select key from src group by key
PREHOOK: type: QUERY
POSTHOOK: query: explain select key from src group by key
POSTHOOK: type: QUERY
ABSTRACT SYNTAX TREE:
  (TOK_QUERY (TOK_FROM (TOK_TABREF (TOK_TABNAME src))) (TOK_INSERT (TOK_DESTINATION (TOK_DIR TOK_TMP_FILE)) (TOK_SELECT (TOK_SELEXPR (TOK_TABLE_OR_COL key))) (TOK_GROUPBY (TOK_TABLE_OR_COL key))))

STAGE DEPENDENCIES:
  Stage-1 is a root stage
  Stage-0 is a root stage

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Alias -> Map Operator Tree:
        src 
          TableScan
            alias: src
            Select Operator
              expressions:
                    expr: key
                    type: string
              outputColumnNames: key
              Group By Operator
                bucketGroup: false
                keys:
                      expr: key
                      type: string
                mode: hash
                outputColumnNames: _col0
                Reduce Output Operator
                  key expressions:
                        expr: _col0
                        type: string
                  sort order: +
                  Map-reduce partition columns:
                        expr: _col0
                        type: string
                  tag: -1
      Reduce Operator Tree:
        Group By Operator
          bucketGroup: false
          keys:
                expr: KEY._col0
                type: string
          mode: mergepartial
          outputColumnNames: _col0
          Select Operator
            expressions:
                  expr: _col0
                  type: string
            outputColumnNames: _col0
            File Output Operator
              compressed: false
              GlobalTableId: 0
              table:
                  input format: org.apache.hadoop.mapred.TextInputFormat
                  output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat

  Stage: Stage-0
    Fetch Operator
      limit: -1