distinct用group by优化

当数据量非常大，在同一个query中计算多个不相关列的distinct时，往往很容易出现数据倾斜现象，导致运行半天都不能得到结果。

比如以下的SQL语句（a, b, c没有相关性）：

select distinct(a), distinct(b), distinct(c) from tableName;

改进方案：把计算的distinct先group by，然后再计算：

select distinct(t.a), distinct(t.b), distinct(t.c)
from(
    select a, b, c from tableName
    group by a, b, c;
)t;

原理：distinct是在reduce里面计算的，再进行merge，而group by的字段会在map这里分N路hash。group by越多，hash分的越随机。

group by中avg以及sum的处理方式：

select
    count(distinct a) as a, count(distinct b) as b,
    count(distinct c) as c, count(distinct d) as d,
    avg(e) as e,
    sum(f) as f
from tablename;

处理方式如下：

select
    count(distinct a) as a, count(distinct b) as b,
    count(distinct c) as c, count(distinct d) as d,
    sum(e)/sum(e_count) as e,  // avg的处理
    sum(f) as f  // sum的处理
from (
    select v.a, v.b, v.c, v.d,
        sum(v.e) as e,
        sum(v.f) as f,
        sum(v.e_count) as e_count
    from(
        select
            a, b, c, d, e, f,
            case when e=null then 0 else 1 end as e_count
        from tablename
    )v
    group by v.a, v.b, v.c, v.d
)t;