0
点赞
收藏
分享

微信扫一扫

Hive中不走MapReduce的查询

ivy吖 2022-12-11 阅读 115


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


[java]  ​​view plain​​  ​​copy​​



  1. select * from employees;  


结果显示如下:


[java]  ​​view plain​​  ​​copy​​



  1. hive> select * from employees;  
  2. OK  
  3. lavimer 15000.0 ["li","lu","wang"]  {"k1":1.0,"k2":2.0,"k3":3.0}    {"street":"dingnan","city":"ganzhou","num":101}  
  4. liao    18000.0 ["liu","li","huang"]    {"k4":2.0,"k5":3.0,"k6":6.0}    {"street":"dingnan","city":"ganzhou","num":102}  
  5. zhang   19000.0 ["xiao","wen","tian"]   {"k7":7.0,"k8":8.0,"k8":8.0}    {"street":"dingnan","city":"ganzhou","num":103}  
  6. Time taken: 0.176 seconds  
  7. 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


[java]  ​​view plain​​  ​​copy​​



  1. 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:


[java]  ​​view plain​​  ​​copy​​



  1. hive> select * from employees limit 2;  
  2. OK  
  3. lavimer 15000.0 ["li","lu","wang"]  {"k1":1.0,"k2":2.0,"k3":3.0}    {"street":"dingnan","city":"ganzhou","num":101}  
  4. liao    18000.0 ["liu","li","huang"]    {"k4":2.0,"k5":3.0,"k6":6.0}    {"street":"dingnan","city":"ganzhou","num":102}  
  5. Time taken: 0.079 seconds  
  6. 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



举报

相关推荐

0 条评论