1.问题描述：
在使用hive客户端，进行分区联合查询时，发现union查询一直不成功
union查询：
select * from test_partitions where day = '0722' union select * from test_partitions where day = '0724';
结果如下，一直停在这里：
Total jobs = 1
Launching Job 1 out of 1
Number of reduce tasks determined at compile time: 1
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>
Interrupting... Be patient, this might take some time.
Press Ctrl+C again to kill JVM
2.后来我发现我的hadoop集群只有hdfs集群起来了，yarn集群并没有开启，因此，我重启了hadoop集群，之后union查询就成功了，但是觉得好慢啊，不知道为什么，
