Hive中不走MapReduce的查询

在Hive中查询语句往往都要被解析成MapReduce的job进行计算，但是有两个查询语句是不走MapReduce的，如下：

1.查询某张表的所有数据

select * from employees;

结果显示如下：

hive> select * from employees;
OK
lavimer	15000.0	["li","lu","wang"]	{"k1":1.0,"k2":2.0,"k3":3.0}	{"street":"dingnan","city":"ganzhou","num":101}
liao	18000.0	["liu","li","huang"]	{"k4":2.0,"k5":3.0,"k6":6.0}	{"street":"dingnan","city":"ganzhou","num":102}
zhang	19000.0	["xiao","wen","tian"]	{"k7":7.0,"k8":8.0,"k8":8.0}	{"street":"dingnan","city":"ganzhou","num":103}
Time taken: 0.176 seconds
hive> 

从上述语句中我们可以发现这个查询语句并没有走MapReduce。

2.抽样查询

select * from employees limit 2;

注：在MYSQL中limit是取前几条记录，但是在Hive中，limit是抽样，会随机返回对应的记录数。

结果显示如下：

hive> select * from employees limit 2;
OK
lavimer	15000.0	["li","lu","wang"]	{"k1":1.0,"k2":2.0,"k3":3.0}	{"street":"dingnan","city":"ganzhou","num":101}
liao	18000.0	["liu","li","huang"]	{"k4":2.0,"k5":3.0,"k6":6.0}	{"street":"dingnan","city":"ganzhou","num":102}
Time taken: 0.079 seconds
hive> 

从上述语句中我们可以发现这个查询语句并没有走MapReduce。