SQL优化例子

如下SQL的优化：

select count(*) from (

select id,name,col1,col2 from t1  where name='xxxx'

union

select id,name ,null as col1,'' as col2 from t2  where id!=1

)

原SQL慢原因：由于union需要过滤 重复记录。所以会有后台的一个group by id,name,col1,col2的操作。

优化：去掉group by。统计出 t1表，t2 表。去除重复记录的次数

优化后：

select sum(a)-sum(dis) from

(

SELECT  count(*) as a ,0 as dis  FROM t1 a where name='xxxx'

union all

SELECT  count(*) as a,0 as dis  FROM  t2 b WHERE status !=8

union all

select 0 as a, count(1) as dis from  t1 a inner join t2 b a.id=b.id and a.name=b.name where a.name='xxxx' and a.col1 is null and a.col2='' and b.status!=8 

) as v