(1)特點
只會開啟乙個reduce做聚合，所以資料量很大的話，很影響效能;如果設定多個reduce但是，輸出還是只有乙個檔案
(2)例項
set mapreduce.job.reduces=2;
insert overwrite local directory '/opt/datas/emp_order' row format delimited fields terminated by '\t' select * from emp order by sal;
儘管設定了兩個reducer個數，但是在目錄下cat /opt/datas/emp_order只有乙個檔案000000_0，因為全域性排序，所以，set mapreduce.job.reduces=2;沒有影響
7369 smith clerk 7902 1980-12-17 800.0 \n 20
7900 james clerk 7698 1981-12-3 950.0 \n 30
7876 adams clerk 7788 1987-5-23 1100.0 \n 20
7521 ward salesman 7698 1981-2-22 1250.0 500.0 30
7654 martin salesman 7698 1981-9-28 1250.0 1400.0 30
7934 miller clerk 7782 1982-1-23 1300.0 \n 10
7844 turner salesman 7698 1981-9-8 1500.0 0.0 30
7499 allen salesman 7698 1981-2-20 1600.0 300.0 30
7782 clark manager 7839 1981-6-9 2450.0 \n 10
7698 blake manager 7839 1981-5-1 2850.0 \n 30
7566 jones manager 7839 1981-4-2 2975.0 \n 20
7788 scott analyst 7566 1987-4-19 3000.0 \n 20
7902 ford analyst 7566 1981-12-3 3000.0 \n 20
7839 king president \n 1981-11-17 5000.0 \n 10
針對每個reduce的結果進行排序，對於分割槽後的reduce進行排序
（2）例項
set mapreduce.job.reduces=2;
insert overwrite local directory '/opt/datas/emp_sort' row format delimited fields terminated by '\t' select * from emp sort by sal;
000000_0 000001_0
[root@bigdata emp_sort]# cat 000000_0
7369 smith clerk 7902 1980-12-17 800.0 \n 20
7900 james clerk 7698 1981-12-3 950.0 \n 30
7876 adams clerk 7788 1987-5-23 1100.0 \n 20
7654 martin salesman 7698 1981-9-28 1250.0 1400.0 30
7521 ward salesman 7698 1981-2-22 1250.0 500.0 30
7844 turner salesman 7698 1981-9-8 1500.0 0.0 30
7566 jones manager 7839 1981-4-2 2975.0 \n 20
7902 ford analyst 7566 1981-12-3 3000.0 \n 20
7788 scott analyst 7566 1987-4-19 3000.0 \n 20
[root@bigdata emp_sort]# cat 000001_0
7934 miller clerk 7782 1982-1-23 1300.0 \n 10
7499 allen salesman 7698 1981-2-20 1600.0 300.0 30
7782 clark manager 7839 1981-6-9 2450.0 \n 10
7698 blake manager 7839 1981-5-1 2850.0 \n 30
7839 king president \n 1981-11-17 5000.0 \n 10
按照指定的字段進行分割槽，然後再做其他操作。
（2）例項
set mapreduce.job.reduces=3;
insert overwrite local directory '/opt/datas/emp_dist' row format delimited fields terminated by '\t' select * from emp distribute by deptno sort by sal;
000000_0 000001_0 000002_0
[root@bigdata emp_dist]# cat 000000_0
7900 james clerk 7698 1981-12-3 950.0 \n 30
7521 ward salesman 7698 1981-2-22 1250.0 500.0 30
7654 martin salesman 7698 1981-9-28 1250.0 1400.0 30
7844 turner salesman 7698 1981-9-8 1500.0 0.0 30
7499 allen salesman 7698 1981-2-20 1600.0 300.0 30
7698 blake manager 7839 1981-5-1 2850.0 \n 30
[root@bigdata emp_dist]# cat 000001_0
7934 miller clerk 7782 1982-1-23 1300.0 \n 10
7782 clark manager 7839 1981-6-9 2450.0 \n 10
7839 king president \n 1981-11-17 5000.0 \n 10
[root@bigdata emp_dist]# cat 000002_0
7369 smith clerk 7902 1980-12-17 800.0 \n 20
7876 adams clerk 7788 1987-5-23 1100.0 \n 20
7566 jones manager 7839 1981-4-2 2975.0 \n 20
7788 scott analyst 7566 1987-4-19 3000.0 \n 20
7902 ford analyst 7566 1981-12-3 3000.0 \n 20
指定分割槽字段，並且按照分割槽字段排序。等於distribute by sal sort by sal，兩者是同樣的字段。但是這個情況應用的很少。
（2）例項
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