为什么在配置单元插入中需要减速器

Question

当我们从配置单元命令行触发insert into语句时，问题与MapReduce作业的工作有关。在将记录插入到hive表中时:由于插入到内部hive表中时没有涉及聚合，因此为什么还会调用reducer。它应该只是一个映射器作业。在这里，reducer的作用是什么。

insert into table values (1,1);

INFO  : Hadoop job information for Stage-1: number of mappers: 1; number of reducers: 1
INFO  : 2021-04-28 10:30:26,487 Stage-1 map = 0%,  reduce = 0%
INFO  : 2021-04-28 10:30:30,604 Stage-1 map = 100%,  reduce = 0%, Cumulative CPU 1.96 sec
INFO  : 2021-04-28 10:30:36,774 Stage-1 map = 100%,  reduce = 100%, Cumulative CPU 3.35 sec
INFO  : MapReduce Total cumulative CPU time: 3 seconds 350 msec

hive> set hive.merge.mapfiles;
hive.merge.mapfiles=true
hive> set hive.merge.mapredfiles;
hive.merge.mapredfiles=false
hive> set mapreduce.job.reduces;
mapreduce.job.reduces=-1

explain insert into test values (10,14);

STAGE DEPENDENCIES:
  Stage-1 is a root stage
  Stage-7 depends on stages: Stage-1 , consists of Stage-4, Stage-3, Stage-5
  Stage-4
  Stage-0 depends on stages: Stage-4, Stage-3, Stage-6
  Stage-2 depends on stages: Stage-0
  Stage-3
  Stage-5
  Stage-6 depends on stages: Stage-5

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Map Operator Tree:
          TableScan
            alias: _dummy_table
            Row Limit Per Split: 1
            Statistics: Num rows: 1 Data size: 10 Basic stats: COMPLETE Column stats: COMPLETE
            Select Operator
              expressions: array(const struct(10,14)) (type: array<struct<col1:int,col2:int>>)
              outputColumnNames: _col0
              Statistics: Num rows: 1 Data size: 48 Basic stats: COMPLETE Column stats: COMPLETE
              UDTF Operator
                Statistics: Num rows: 1 Data size: 48 Basic stats: COMPLETE Column stats: COMPLETE
                function name: inline
                Select Operator
                  expressions: col1 (type: int), col2 (type: int)
                  outputColumnNames: _col0, _col1
                  Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: COMPLETE
                  File Output Operator
                    compressed: false
                    Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: COMPLETE
                    table:
                        input format: org.apache.hadoop.mapred.TextInputFormat
                        output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
                        serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe
                  Select Operator
                    expressions: _col0 (type: int), _col1 (type: int)
                    outputColumnNames: i, j
                    Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: COMPLETE
                    Group By Operator
                      aggregations: compute_stats(i, 'hll'), compute_stats(j, 'hll')
                      mode: hash
                      outputColumnNames: _col0, _col1
                      Statistics: Num rows: 1 Data size: 848 Basic stats: COMPLETE Column stats: COMPLETE
                      Reduce Output Operator
                        sort order: 
                        Statistics: Num rows: 1 Data size: 848 Basic stats: COMPLETE Column stats: COMPLETE
                        value expressions: _col0 (type: struct<columntype:string,min:bigint,max:bigint,countnulls:bigint,bitvector:binary>), _col1 (type: struct<columntype:string,min:bigint,max:bigint,countnulls:bigint,bitvector:binary>)
      Reduce Operator Tree:
        Group By Operator
          aggregations: compute_stats(VALUE._col0), compute_stats(VALUE._col1)
          mode: mergepartial
          outputColumnNames: _col0, _col1
          Statistics: Num rows: 1 Data size: 880 Basic stats: COMPLETE Column stats: COMPLETE
          File Output Operator
            compressed: false
            Statistics: Num rows: 1 Data size: 880 Basic stats: COMPLETE Column stats: COMPLETE
            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

  Stage: Stage-7
    Conditional Operator

  Stage: Stage-4
    Move Operator
      files:
          hdfs directory: true
          destination:<path>
  Stage: Stage-0
    Move Operator
      tables:
          replace: false
          table:
              input format: org.apache.hadoop.mapred.TextInputFormat
              output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
              serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe

  Stage: Stage-2
    Stats Work
      Basic Stats Work:
      Column Stats Desc:
          Columns: i, j
          Column Types: int, int
          Table: db.test.test

  Stage: Stage-3
    Map Reduce
      Map Operator Tree:
          TableScan
            File Output Operator
              compressed: false
              table:
                  input format: org.apache.hadoop.mapred.TextInputFormat
                  output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
                  serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe
                  name: db.test

  Stage: Stage-5
    Map Reduce
      Map Operator Tree:
          TableScan
            File Output Operator
              compressed: false
              table:
                  input format: org.apache.hadoop.mapred.TextInputFormat
                  output format: org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat
                  serde: org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe
                  name: db.test

  Stage: Stage-6
    Move Operator
      files:
          hdfs directory: true
          destination: <PATH>
          
Time taken: 5.123 seconds, Fetched: 121 row(s)

Answer 1

您似乎启用了统计信息自动收集功能：

SET hive.stats.autogather=true;

而reducer正在计算统计数据

Reduce Operator Tree:
        Group By Operator
          aggregations: **compute_stats**(VALUE._col0), compute_stats(VALUE._col1)
          mode: mergepartial