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                            數據量小的時候無所謂,數據量大的情況下,由于count distinct操作需要用一個Reduce Task來完成,這一個Reduce需要處理的數據量太大,就會導致整個Job很難完成.

一般count distinct使用先group by再count的方式替換:

原本sql
~~~
select count(distinct id) from bigtable;
~~~

采用group by 去重id

~~~
select count(id) from (select id from bigtable group by id) a;
~~~

雖然時間變長,但是reduce節點負載差不多了
                    
                
        
                
            
        
                
    
                
    
                
        
                
    
                

                







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