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                **1. `distribute by`特點** (1)<mark>先按`distribute by`進行分區,然后再使用`sort by`進行排序。相當于先在分區之間對分區進行排序,然后再對每個分區內的數據進行排序</mark>。 (2)Hive 要求`distribute by` 語句要寫在 `sort by` 語句之前。 (3)對于 `distribute by` 進行測試,一定要分配多 reduce 進行處理,否則無法看到`distribute by` 的效果。可以根據相應列以及對應 reduce 的個數進行分發。 (4)默認是采用 hash 算法進行分區。`$ 分區id=分區字段值的 hash 碼 % reduce 的個數$`。 <br/> **2. `distribute by`使用示例** ```sql -- 設置 reduce 個數 0: jdbc:hive2://hadoop101:10000> set mapreduce.job.reduces=3; -- 查看設置 reduce 個數 0: jdbc:hive2://hadoop101:10000> set mapreduce.job.reduces; +--------------------------+--+ | set | +--------------------------+--+ | mapreduce.job.reduces=3 | +--------------------------+--+ -- 先使用deptno進行distribute by分區 -- 后用empno進行sort by降序排序 select * from emp distribute by deptno sort by empno desc; +------------+------------+------------+----------+---------------+----------+-----------+-------------+--+ | emp.empno | emp.ename | emp.job | emp.mgr | emp.hiredate | emp.sal | emp.comm | emp.deptno | +------------+------------+------------+----------+---------------+----------+-----------+-------------+--+ | 7900 | JAMES | CLERK | 7698 | 1981-12-3 | 950.0 | NULL | 30 | | 7844 | TURNER | SALESMAN | 7698 | 1981-9-8 | 1500.0 | 0.0 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-5-1 | 2850.0 | NULL | 30 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-9-28 | 1250.0 | 1400.0 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-2-22 | 1250.0 | 500.0 | 30 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-2-20 | 1600.0 | 300.0 | 30 | | 7934 | MILLER | CLERK | 7782 | 1982-1-23 | 1300.0 | NULL | 10 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.0 | NULL | 10 | | 7782 | CLARK | MANAGER | 7839 | 1981-6-9 | 2450.0 | NULL | 10 | ```
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