hive中count(distinct) 的原理

参考博客：

https://blog.csdn.net/oracle8090/article/details/80760233

https://www.cnblogs.com/ling1995/p/7339424.html（看我）

count（distinct id）的原理

count（distinct id）从执行计划上面来看：只有一个reducer任务（即使你设置reducer任务为100个，实际上也没有用），所有的id都

会聚集到同一个reducer任务进行去重然后在聚合，这非常容易造成数据倾斜.

STAGE DEPENDENCIES:
  Stage-1 is a root stage
  Stage-0 depends on stages: Stage-1

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Map Operator Tree:
          TableScan
            alias: emp_ct
            Statistics: Num rows: 42 Data size: 171 Basic stats: COMPLETE Column stats: NONE
            Select Operator
              expressions: dept_num (type: int)
              outputColumnNames: _col0
              Statistics: Num rows: 42 Data size: 171 Basic stats: COMPLETE Column stats: NONE
              Reduce Output Operator
                key expressions: _col0 (type: int)
                sort order: +
                Statistics: Num rows: 42 Data size: 171 Basic stats: COMPLETE Column stats: NONE
      Reduce Operator Tree:
        Group By Operator
          aggregations: count(DISTINCT KEY._col0:0._col0)
          mode: complete
          outputColumnNames: _col0
          Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
          File Output Operator
            compressed: false
            Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
            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-0
    Fetch Operator
      limit: -1
      Processor Tree:
        ListSink

　　

运行示例：注意设置的reducer任务数量实际上是不生效的。

hive> set mapreduce.job.reduces=5;
hive> 
    > select count(distinct dept_num)
    > from emp_ct;
Query ID = mart_fro_20200320233947_4f60c190-4967-4da6-bf3e-97db786fbc6c
Total jobs = 1
Launching Job 1 out of 1
Number of reduce tasks determined at compile time: 1
In order to change the average load for a reducer (in bytes):
  set hive.exec.reducers.bytes.per.reducer=<number>
In order to limit the maximum number of reducers:
  set hive.exec.reducers.max=<number>
In order to set a constant number of reducers:
  set mapreduce.job.reduces=<number>
Start submit job !
Start GetSplits
GetSplits finish, it costs : 32 milliseconds
Submit job success : job_1584341089622_358496
Starting Job = job_1584341089622_358496, Tracking URL = http://BJHTYD-Hope-25-11.hadoop.jd.local:50320/proxy/application_1584341089622_358496/
Kill Command = /data0/hadoop/hadoop_2.100.31_2019090518/bin/hadoop job  -kill job_1584341089622_358496
Hadoop job(job_1584341089622_358496) information for Stage-1: number of mappers: 2; number of reducers: 1
2020-03-20 23:39:58,215 Stage-1(job_1584341089622_358496) map = 0%,  reduce = 0%
2020-03-20 23:40:09,628 Stage-1(job_1584341089622_358496) map = 50%,  reduce = 0%, Cumulative CPU 2.74 sec
2020-03-20 23:40:16,849 Stage-1(job_1584341089622_358496) map = 100%,  reduce = 0%, Cumulative CPU 7.43 sec
2020-03-20 23:40:29,220 Stage-1(job_1584341089622_358496) map = 100%,  reduce = 100%, Cumulative CPU 10.64 sec
MapReduce Total cumulative CPU time: 10 seconds 640 msec
Stage-1  Elapsed : 40533 ms  job_1584341089622_358496
Ended Job = job_1584341089622_358496
MapReduce Jobs Launched: 
Stage-1: Map: 2  Reduce: 1   Cumulative CPU: 10.64 sec   HDFS Read: 0.000 GB HDFS Write: 0.000 GB SUCCESS  Elapsed : 40s533ms job_1584341089622_358496

Total MapReduce CPU Time Spent: 10s640ms
Total Map: 2  Total Reduce: 1
Total HDFS Read: 0.000 GB  Written: 0.000 GB
OK
3
Time taken: 43.025 seconds, Fetched: 1 row(s)

　　

count（distinct id）的解决方案

该怎么解决这个问题呢？实际上解决方法非常巧妙：

我们利用Hive对嵌套语句的支持，将原来一个MapReduce作业转换为两个作业，在第一阶段选出全部的非重复id，在第二阶段再对

这些已消重的id进行计数。这样在第一阶段我们可以通过增大Reduce的并发数，并发处理Map输出。在第二阶段，由于id已经消重，

因此COUNT(*)操作在Map阶段不需要输出原id数据，只输出一个合并后的计数即可。这样即使第二阶段Hive强制指定一个Reduce Task，

极少量的Map输出数据也不会使单一的Reduce Task成为瓶颈。改进后的SQL语句如下：

查看一下执行计划：

STAGE DEPENDENCIES:
  Stage-1 is a root stage
  Stage-2 depends on stages: Stage-1
  Stage-0 depends on stages: Stage-2

STAGE PLANS:
  Stage: Stage-1
    Map Reduce
      Map Operator Tree:
          TableScan
            alias: emp_ct
            Statistics: Num rows: 42 Data size: 171 Basic stats: COMPLETE Column stats: NONE
            Select Operator
              expressions: dept_num (type: int)
              outputColumnNames: dept_num
              Statistics: Num rows: 42 Data size: 171 Basic stats: COMPLETE Column stats: NONE
              Reduce Output Operator
                key expressions: dept_num (type: int)
                sort order: +
                Map-reduce partition columns: dept_num (type: int)
                Statistics: Num rows: 42 Data size: 171 Basic stats: COMPLETE Column stats: NONE
      Reduce Operator Tree:
        Group By Operator
          keys: KEY._col0 (type: int)
          mode: complete
          outputColumnNames: _col0
          Statistics: Num rows: 21 Data size: 85 Basic stats: COMPLETE Column stats: NONE
          File Output Operator
            compressed: false
            table:
                input format: org.apache.hadoop.mapred.SequenceFileInputFormat
                output format: org.apache.hadoop.hive.ql.io.HiveSequenceFileOutputFormat
                serde: org.apache.hadoop.hive.serde2.lazybinary.LazyBinarySerDe

  Stage: Stage-2
    Map Reduce
      Map Operator Tree:
          TableScan
            Reduce Output Operator
              sort order: 
              Statistics: Num rows: 21 Data size: 85 Basic stats: COMPLETE Column stats: NONE
              value expressions: _col0 (type: int)
      Reduce Operator Tree:
        Group By Operator
          aggregations: count(VALUE._col0)
          mode: complete
          outputColumnNames: _col0
          Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
          File Output Operator
            compressed: false
            Statistics: Num rows: 1 Data size: 8 Basic stats: COMPLETE Column stats: NONE
            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-0
    Fetch Operator
      limit: -1
      Processor Tree:
        ListSink

具体看一下执行结果：注意看reducer任务的数量，第一个reducer任务是5个，第二个是1个.

hive> set mapreduce.job.reduces=5;
hive> 
    > select count(dept_num)
    > from (
    >        select distinct dept_num
    >        from emp_ct
    >        ) t1;
Query ID = mart_fro_20200320234453_68ad3780-c3e5-44bc-94df-58a8f2b01f59
Total jobs = 2
Launching Job 1 out of 2
Number of reduce tasks not specified. Defaulting to jobconf value of: 5
In order to change the average load for a reducer (in bytes):
  set hive.exec.reducers.bytes.per.reducer=<number>
In order to limit the maximum number of reducers:
  set hive.exec.reducers.max=<number>
In order to set a constant number of reducers:
  set mapreduce.job.reduces=<number>
Start submit job !
Start GetSplits
GetSplits finish, it costs : 13 milliseconds
Submit job success : job_1584341089622_358684
Starting Job = job_1584341089622_358684, Tracking URL = http://BJHTYD-Hope-25-11.hadoop.jd.local:50320/proxy/application_1584341089622_358684/
Kill Command = /data0/hadoop/hadoop_2.100.31_2019090518/bin/hadoop job  -kill job_1584341089622_358684
Hadoop job(job_1584341089622_358684) information for Stage-1: number of mappers: 2; number of reducers: 5
2020-03-20 23:45:02,920 Stage-1(job_1584341089622_358684) map = 0%,  reduce = 0%
2020-03-20 23:45:23,533 Stage-1(job_1584341089622_358684) map = 50%,  reduce = 0%, Cumulative CPU 3.48 sec
2020-03-20 23:45:25,596 Stage-1(job_1584341089622_358684) map = 100%,  reduce = 0%, Cumulative CPU 7.08 sec
2020-03-20 23:45:32,804 Stage-1(job_1584341089622_358684) map = 100%,  reduce = 20%, Cumulative CPU 9.43 sec
2020-03-20 23:45:34,861 Stage-1(job_1584341089622_358684) map = 100%,  reduce = 40%, Cumulative CPU 12.39 sec
2020-03-20 23:45:36,923 Stage-1(job_1584341089622_358684) map = 100%,  reduce = 80%, Cumulative CPU 18.47 sec
2020-03-20 23:45:40,011 Stage-1(job_1584341089622_358684) map = 100%,  reduce = 100%, Cumulative CPU 23.23 sec
MapReduce Total cumulative CPU time: 23 seconds 230 msec
Stage-1  Elapsed : 46404 ms  job_1584341089622_358684
Ended Job = job_1584341089622_358684
Launching Job 2 out of 2
Number of reduce tasks determined at compile time: 1
In order to change the average load for a reducer (in bytes):
  set hive.exec.reducers.bytes.per.reducer=<number>
In order to limit the maximum number of reducers:
  set hive.exec.reducers.max=<number>
In order to set a constant number of reducers:
  set mapreduce.job.reduces=<number>
Start submit job !
Start GetSplits
GetSplits finish, it costs : 47 milliseconds
Submit job success : job_1584341089622_358729
Starting Job = job_1584341089622_358729, Tracking URL = http://BJHTYD-Hope-25-11.hadoop.jd.local:50320/proxy/application_1584341089622_358729/
Kill Command = /data0/hadoop/hadoop_2.100.31_2019090518/bin/hadoop job  -kill job_1584341089622_358729
Hadoop job(job_1584341089622_358729) information for Stage-2: number of mappers: 5; number of reducers: 1
2020-03-20 23:45:48,353 Stage-2(job_1584341089622_358729) map = 0%,  reduce = 0%
2020-03-20 23:46:05,846 Stage-2(job_1584341089622_358729) map = 20%,  reduce = 0%, Cumulative CPU 2.62 sec
2020-03-20 23:46:06,873 Stage-2(job_1584341089622_358729) map = 60%,  reduce = 0%, Cumulative CPU 8.49 sec
2020-03-20 23:46:08,931 Stage-2(job_1584341089622_358729) map = 80%,  reduce = 0%, Cumulative CPU 11.53 sec
2020-03-20 23:46:09,960 Stage-2(job_1584341089622_358729) map = 100%,  reduce = 0%, Cumulative CPU 15.23 sec
2020-03-20 23:46:35,639 Stage-2(job_1584341089622_358729) map = 100%,  reduce = 100%, Cumulative CPU 20.37 sec
MapReduce Total cumulative CPU time: 20 seconds 370 msec
Stage-2  Elapsed : 54552 ms  job_1584341089622_358729
Ended Job = job_1584341089622_358729
MapReduce Jobs Launched: 
Stage-1: Map: 2  Reduce: 5   Cumulative CPU: 23.23 sec   HDFS Read: 0.000 GB HDFS Write: 0.000 GB SUCCESS  Elapsed : 46s404ms job_1584341089622_358684

Stage-2: Map: 5  Reduce: 1   Cumulative CPU: 20.37 sec   HDFS Read: 0.000 GB HDFS Write: 0.000 GB SUCCESS  Elapsed : 54s552ms job_1584341089622_358729

Total MapReduce CPU Time Spent: 43s600ms
Total Map: 7  Total Reduce: 6
Total HDFS Read: 0.000 GB  Written: 0.000 GB
OK
3
Time taken: 103.692 seconds, Fetched: 1 row(s)

　　

这个解决方案有点类似于set hive.groupby.skew.indata参数的作用！

实际测试：

select count(distinct dept_num)
from emp_ct

  
select count(*)
from (
	select distinct dept_num 
	from emp_ct 
)