hive sql的优化

1、count distinct 优化 

--优化前 
select count(distinct id )from tablename 
--优化后 
select count(1) from (select distinct id from tablename)tmp; 
select count(1) from (select id from tablename group by id)tmp;
--注意，如果id 存在空值，那么优化后的两条sql必须写为count(id)，否者count会多一条。count(1)和count(字段)的区别。