Hive，order by ,distribute by ,sort by ,cluster by 作用与区别（转载）

女爷i 2021-09-25 02:36 246阅读 0赞

*具有相同* Distribute By *列的所有行将进入相同的 reducer*  
[https://www.docs4dev.com/docs/zh/apache-hive/3.1.1/reference/LanguageManual\_SortBy.html][https_www.docs4dev.com_docs_zh_apache-hive_3.1.1_reference_LanguageManual_SortBy.html]

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1、order by

hive中的order by 会对查询结果集执行一个全局排序，这也就是说所有的数据都通过一个reduce进行处理的过程，对于大数据集，这个过程将消耗很大的时间来执行。

![e709bcf9883dfbf69b233c4478a2c956.png][]

2、sort by

hive的sort by 也就是执行一个局部排序过程。这可以保证每个reduce的输出数据都是有序的(但并非全局有效)。这样就可以提高后面进行的全局排序的效率了。对于这两种情况，语法区别仅仅是，一个关键字是order，另一个关键字是sort。用户可以指定任意期望进行排序的字段，并可以在字段后面加上asc关键字(默认)表示升序，desc关键字是降序排序。

在使用sort by之前，需要先设置Reduce的数量>1，才会做局部排序，如果Reduce数量是1，作用与order by一样，全局排序。

![1c736e1f3732d20227dca630201e1cd4.png][]

3、distribute by

distribute by 控制 map的输出在reduer中是如何划分的，mapreduce job 中传输的所有数据都是按照键-值对的方式进行组织的，因此hive在将用户的查询语句转换成mapreduce job时，其必须在内部使用这个功能。默认情况下，MapReduce计算框架会依据map输入的键计算相应的哈希值，然后按照得到的哈希值将键-值对均匀分发到多个reducer中去，不过不幸的是，这也是意味着当我们使用sort by 时，不同reducer的输出内容会有明显的重叠，至少对于排序顺序而已只这样，即使每个reducer的输出的数据都有序的。如果我们想让同一年的数据一起处理，那么就可以使用distribute by 来保证具有相同年份的数据分发到同一个reducer中进行处理，然后使用sort by 来安装我们的期望对数据进行排序:

![80e0e3af6ac545dea746eda31ae4c079.png][]

4、cluster by

cluster by 除了distribute by 的功能外，还会对该字段进行排序，所以cluster by = distribute by +sort by 。

eg：select \* from table cluster by year;

等价于：select \* from table distribute by year sort by year;

![1452c2a2aca8ee2c57d19a87c80b138a.png][]

[https://zhuanlan.zhihu.com/p/93747613][https_zhuanlan.zhihu.com_p_93747613]

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hive中的distribute by是控制在map端如何拆分数据给reduce端的。  
hive会根据distribute by后面列，根据reduce的个数进行数据分发，默认是采用hash算法。

对于distribute by进行测试，一定要分配多reduce进行处理，否则无法看到distribute by的效果。

hive> select \* from test09;  
OK  
100 tom  
200 mary  
300 kate  
400 tim  
Time taken: 0.061 seconds

hive> insert overwrite local directory ‘/home/hjl/sunwg/ooo’ select \* from test09 distribute by id;  
Total MapReduce jobs = 1  
Launching Job 1 out of 1  
Number of reduce tasks not specified. Defaulting to jobconf value of: 2  
In order to change the average load for a reducer (in bytes):  
set hive.exec.reducers.bytes.per.reducer=  
In order to limit the maximum number of reducers:  
set hive.exec.reducers.max=  
In order to set a constant number of reducers:  
set mapred.reduce.tasks=  
Starting Job = job\_201105020924\_0070, Tracking URL = http://hadoop00:50030/jobdetails.jsp?jobid=job\_201105020924\_0070  
Kill Command = /home/hjl/hadoop/bin/../bin/hadoop job -Dmapred.job.tracker=hadoop00:9001 -kill job\_201105020924\_0070  
2011-05-03 06:12:36,644 Stage-1 map = 0%, reduce = 0%  
2011-05-03 06:12:37,656 Stage-1 map = 50%, reduce = 0%  
2011-05-03 06:12:39,673 Stage-1 map = 100%, reduce = 0%  
2011-05-03 06:12:44,713 Stage-1 map = 100%, reduce = 50%  
2011-05-03 06:12:46,733 Stage-1 map = 100%, reduce = 100%  
Ended Job = job\_201105020924\_0070  
Copying data to local directory /home/hjl/sunwg/ooo  
Copying data to local directory /home/hjl/sunwg/ooo  
4 Rows loaded to /home/hjl/sunwg/ooo  
OK  
Time taken: 17.663 seconds

第一次执行根据id字段来做分发，结果如下：

\[hjl@sunwg src\]$ cat /home/hjl/sunwg/ooo/attempt\_201105020924\_0070\_r\_000000\_0  
400tim  
200mary  
\[hjl@sunwg src\]$ cat /home/hjl/sunwg/ooo/attempt\_201105020924\_0070\_r\_000001\_0  
300kate  
100tom

这次我们换个分发的方式，采用length(id)的结果，因为这几条记录的id字段的长度都相同，所以应该会被分布到同一个reduce中。

hive> insert overwrite local directory ‘/home/hjl/sunwg/lll’ select \* from test09 distribute by length(id);  
Total MapReduce jobs = 1  
Launching Job 1 out of 1  
Number of reduce tasks not specified. Defaulting to jobconf value of: 2  
In order to change the average load for a reducer (in bytes):  
set hive.exec.reducers.bytes.per.reducer=  
In order to limit the maximum number of reducers:  
set hive.exec.reducers.max=  
In order to set a constant number of reducers:  
set mapred.reduce.tasks=  
Starting Job = job\_201105020924\_0071, Tracking URL = http://hadoop00:50030/jobdetails.jsp?jobid=job\_201105020924\_0071  
Kill Command = /home/hjl/hadoop/bin/../bin/hadoop job -Dmapred.job.tracker=hadoop00:9001 -kill job\_201105020924\_0071  
2011-05-03 06:15:21,430 Stage-1 map = 0%, reduce = 0%  
2011-05-03 06:15:24,454 Stage-1 map = 100%, reduce = 0%  
2011-05-03 06:15:31,509 Stage-1 map = 100%, reduce = 50%  
2011-05-03 06:15:34,539 Stage-1 map = 100%, reduce = 100%  
Ended Job = job\_201105020924\_0071  
Copying data to local directory /home/hjl/sunwg/lll  
Copying data to local directory /home/hjl/sunwg/lll  
4 Rows loaded to /home/hjl/sunwg/lll  
OK  
Time taken: 20.632 seconds

在查看下结果是否和我们的预期相同：  
\[hjl@sunwg src\]$ cat /home/hjl/sunwg/lll/attempt\_201105020924\_0071\_r\_000000\_0  
\[hjl@sunwg src\]$ cat /home/hjl/sunwg/lll/attempt\_201105020924\_0071\_r\_000001\_0  
100tom  
200mary  
300kate  
400tim

文件attempt\_201105020924\_0071\_r\_000000\_0中没有记录，而全部的记录都在attempt\_201105020924\_0071\_r\_000001\_0中。

[https://blog.csdn.net/cjlion/article/details/80879469][https_blog.csdn.net_cjlion_article_details_80879469]

[https_www.docs4dev.com_docs_zh_apache-hive_3.1.1_reference_LanguageManual_SortBy.html]: https://www.docs4dev.com/docs/zh/apache-hive/3.1.1/reference/LanguageManual_SortBy.html
[e709bcf9883dfbf69b233c4478a2c956.png]: /images/20210923/674d8409605d4d92aec94735e2997161.png
[1c736e1f3732d20227dca630201e1cd4.png]: /images/20210923/4f37a3daffea46419ce9796746b27f04.png
[80e0e3af6ac545dea746eda31ae4c079.png]: /images/20210923/2ec1beba4c1a40f89b787188f3a19ea8.png
[1452c2a2aca8ee2c57d19a87c80b138a.png]: /images/20210923/3c3fade53bfb4279b54485bcc518d213.png
[https_zhuanlan.zhihu.com_p_93747613]: https://zhuanlan.zhihu.com/p/93747613
[https_blog.csdn.net_cjlion_article_details_80879469]: https://blog.csdn.net/cjlion/article/details/80879469