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。

其实，为了查询效率，简单查询hive就不走mr，这个是可以设置的，在hive-site.xml里面有个配置参数叫：

hive.fetch.task.conversion

将这个参数设置为more，简单查询就不走map/reduce了，设置为minimal，就任何简单select都会走map/reduce