hive 之with as 和create view 和create temporary table用法

create temporary table test.cc_tmp as select * from test.cc_join where name like '%c%';

explain
select * from test.cc_tmp where id >0
union all
select * from test.cc_tmp where id is null ;

create view test.cc_tmp_v as select * from test.cc_join where name like '%c%'

explain
select * from test.cc_tmp_v where id >0
union all
select * from test.cc_tmp_v where id is null ;

explain with tmp as
(select * from test.cc_join where name like '%c%')

select * from tmp where id >0
union all
select * from tmp where id is null

三者

create temporary table 是创建一张临时表，退出当前会话，数据就不见了

通过show create table 可以看到是放在tmp目录的。

create view 其实和with tmp as 很相似，都是把复杂的可以重用的sql简化，我觉得唯一的区别就是 view是可以创建下次再使用的 但是with只是当前sql有效，甚至不是会话有效。

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之前网上不知道看的那篇文章说with里面的内容可以复用，也就是只计算一次 下次就不计算了，直接拿上次的结果。放屁。

贴上 view 和with的explain 可以看到 都是经过了两次计算

而create temporary才是真正的一次计算

突然觉得这个例子不好。换个例子

create temporary table test.cc_tmp as select a.id,b.name from test.cc_join a join test.cc_join b on a.id =b.id

select * from test.cc_tmp
union all
select * from test.cc_tmp

create view test.cc_tmp_v as select a.id,b.name from test.cc_join a join test.cc_join b on a.id =b.id

explain
select * from test.cc_tmp_v
union all
select * from test.cc_tmp_v

with tmp as
(
select a.id,b.name from test.cc_join a join test.cc_join b on a.id =b.id)
select * from tmp
union all
select * from tmp

可以看到 这个join with as 和 view 都是执行了两次，但是temporary table 只执行了一次join，所以如果以后尽量不要用with as 卵用没有。

使用temporary table 感觉才是优解