我们学习一下分桶表，其实分区和分桶这两个概念对于初学者来说是比较难理解的。但对于理解了的人来说，发现又是如此简单。

我们先建立一个分桶表，并尝试直接上传一个数据

create table student4(sno int,sname string,sex string,sage int, sdept string) clustered by(sno) into 3 buckets row format delimited fields terminated by ',';
set hive.enforce.bucketing = true;强制分桶。
load data local inpath '/home/hadoop/hivedata/students.txt' overwrite into table student4;

我们看到虽然设置了强制分桶，但实际student表下面只有一个students一个文件。分桶也就是分区，分区数量等于文件数，所以上面方法并没有分桶。

现在，我们用插入的方法给另外一个分桶表传入同样数据

create table student4(sno int,sname string,sex string,sage int, sdept string) clustered by(sno) into 3 buckets row format delimited fields terminated by ',';
set hive.enforce.bucketing = true;强制分桶。
load data local inpath '/home/hadoop/hivedata/students.txt' overwrite into table student4;
我们看到虽然设置了强制分桶，但实际STUDENT表下面只有一个STUDENTS一个文件。
分桶也就是分区，分区数量等于文件数，所以上面方法并没有分桶。
#创建第2个分桶表
create table stu_buck(sno int,sname string,sex string,sage int,sdept string)
clustered by(sno) 
sorted by(sno DESC)
into 4 buckets
row format delimited
fields terminated by ',';

#设置变量,设置分桶为true, 设置reduce数量是分桶的数量个数
set hive.enforce.bucketing = true;
set mapreduce.job.reduces=4;
#开会往创建的分通表插入数据(插入数据需要是已分桶, 且排序的)
#可以使用distribute by(sno) sort by(sno asc)   或是排序和分桶的字段相同的时候使用Cluster by(字段)
#注意使用cluster by  就等同于分桶+排序(sort)
insert into table stu_buck
select sno,sname,sex,sage,sdept from student distribute by(sno) sort by(sno asc);

Query ID = root_20171109145012_7088af00-9356-46e6-a988-f1fc5f6d2e13
Total jobs = 1
Launching Job 1 out of 1
Number of reduce tasks determined at compile time: 4
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>
Starting Job = job_1510197346181_0014, Tracking URL = http://server71:8088/proxy/application_1510197346181_0014/
Kill Command = /usr/local/hadoop/bin/hadoop job  -kill job_1510197346181_0014
Hadoop job information for Stage-1: number of mappers: 1; number of reducers: 4
2017-11-09 14:50:59,642 Stage-1 map = 0%,  reduce = 0%
2017-11-09 14:51:38,682 Stage-1 map = 100%,  reduce = 0%, Cumulative CPU 5.04 sec
2017-11-09 14:52:31,935 Stage-1 map = 100%,  reduce = 50%, Cumulative CPU 7.91 sec
2017-11-09 14:52:33,467 Stage-1 map = 100%,  reduce = 67%, Cumulative CPU 15.51 sec
2017-11-09 14:52:39,420 Stage-1 map = 100%,  reduce = 83%, Cumulative CPU 22.5 sec
2017-11-09 14:52:40,953 Stage-1 map = 100%,  reduce = 92%, Cumulative CPU 25.86 sec
2017-11-09 14:52:42,243 Stage-1 map = 100%,  reduce = 100%, Cumulative CPU 28.01 sec
MapReduce Total cumulative CPU time: 28 seconds 10 msec
Ended Job = job_1510197346181_0014
Loading data to table default.stu_buck
Table default.stu_buck stats: [numFiles=4, numRows=22, totalSize=527, rawDataSize=505]
MapReduce Jobs Launched: 
Stage-Stage-1: Map: 1  Reduce: 4   Cumulative CPU: 28.01 sec   HDFS Read: 18642 HDFS Write: 819 SUCCESS
Total MapReduce CPU Time Spent: 28 seconds 10 msec
OK
Time taken: 153.794 seconds 

我们设置reduce的数量为4，学过mapreduce的人应该知道reduce数等于分区数，也等于处理的文件数量。