Spark SQL与Hive On MapReduce速度比较

我们都知道Spark比Hadoop的MR计算速度更快。到底快多少呢？我一直比较疑惑，会有官网说的那么夸张吗。

今天就拿基于Spark的Spark SQL和基于MR的Hive比较一下，因为Spark SQL也兼容了HiveQL，我们就可以通过运行相同的HiveQL语句，比较直观的看出到底快多少了。

Spark SQL只要在编译的时候引入Hive支持，就可以支持Hive表访问，UDF，SerDe，以及HiveQL/HQL--引自《Spark快速大数据分析》

Hive一般在工作站上运行，它把SQL查询转化为一系列在Hadoop集群上运行的MR作业--引自《Hadoop权威指南》

用的文件系统都是HDFS，比较的是第二条sql语句

1.运行spark-sql shell

guo@drguo:/opt/spark-1.6.1-bin-hadoop2.6/bin$ spark-sql 

spark-sql> create external table cn(x bigint, y bigint, z bigint, k bigint) 
         > row format delimited fields terminated by ',' 
         > location '/cn';
OK
Time taken: 0.876 seconds
spark-sql> create table p as select y, z, sum(k) as t
         > from cn where x>=20141228 and x<=20150110 group by y, z;
Time taken: 20.658 seconds 

查看jobs，用了15s，为什么上面显示花了接近21s？我也不清楚，先放下不管。

再看一下它的DAG图

2.运行Hive

guo@drguo:~$ hive

hive> create external table cn(x bigint, y bigint, z bigint, k bigint) 
    > row format delimited fields terminated by ',' 
    > location '/cn';
OK
Time taken: 0.752 seconds
hive> create table p as select y, z, sum(k) as t
    > from cn where x>=20141228 and x<=20150110 group by y, z;
Query ID = guo_20160515161032_2e035fc2-6214-402a-90dd-7acda7d638bf
Total jobs = 1
Launching Job 1 out of 1
Number of reduce tasks not specified. Estimated from input data size: 1
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 mapreduce.job.reduces=
Starting Job = job_1463299536204_0001, Tracking URL = http://drguo:8088/proxy/application_1463299536204_0001/
Kill Command = /opt/Hadoop/hadoop-2.7.2/bin/hadoop job  -kill job_1463299536204_0001
Hadoop job information for Stage-1: number of mappers: 1; number of reducers: 1
2016-05-15 16:10:52,353 Stage-1 map = 0%,  reduce = 0%
2016-05-15 16:11:03,418 Stage-1 map = 100%,  reduce = 0%, Cumulative CPU 8.65 sec
2016-05-15 16:11:13,257 Stage-1 map = 100%,  reduce = 100%, Cumulative CPU 13.99 sec
MapReduce Total cumulative CPU time: 13 seconds 990 msec
Ended Job = job_1463299536204_0001
Moving data to: hdfs://drguo:9000/user/hive/warehouse/predict
Table default.predict stats: [numFiles=1, numRows=1486, totalSize=15256, rawDataSize=13770]
MapReduce Jobs Launched: 
Stage-Stage-1: Map: 1  Reduce: 1   Cumulative CPU: 13.99 sec   HDFS Read: 16551670 HDFS Write: 15332 SUCCESS
Total MapReduce CPU Time Spent: 13 seconds 990 msec
OK
Time taken: 42.887 seconds

花了接近43s。再去RM看一下，花了36s

结果非常明显

如果按shell里显示花的时间算，21：43

按网页里显示花的时间算，15：36

差不多一倍，据说即将发布的Spark2.0速度更快。

那么问题来了，Spark什么时候会完全取代MR？