交叉连接
class stack （参见代码示例）将主表行x3相乘，每行一行 class ，然后使用 case 派生列所依赖的语句 class 价值观。带有小数据集（3行）的交叉连接应转换为Map连接，并在Map程序上执行得非常快。

set hive.auto.convert.join=true; --this enables map-join

select t.Name,
       s.class,
       case s.class when 'A' then t.Foo_A 
                    when 'B' then t.foo_B
                    when 'C' then t.foo_C
        end as FooVal,
       case s.class when 'A' then t.Bar_A 
                    when 'B' then t.Bar_B
                    when 'C' then t.Bar_C
        end as BarVal              
 from table t 
      cross join (select stack(3,'A','B','C') as class) s
;

它将只扫描一次表，并且比union all方法的性能要好得多。

配置单元取消拆分多个列：

select
        t1.ID,
        lv.key         as class,
        lv.FooStr.col1 as FooVal,
        lv.FooStr.col2 as BarVal
    from
        table t1
        LATERAL VIEW explode (
            map(
               'A', named_struct('col1', Foo_A, 'col2', Bar_A),
               'B', named_struct('col1', Foo_B, 'col2', Bar_B),
               'C', named_struct('col1', Foo_C, 'col2', Bar_C)
                )) lv as key, FooStr
    where
        coalesce(lv.FooStr.col1, lv.FooStr.col2) IS NOT NULL

谢谢回复！请在下面找到另一种比交叉连接更快的方法。

select t1.ID, t2.key_1 as class, t2.FooVal, t3.BarVal
    from table t1
    LATERAL VIEW explode (map(
   'A', Foo_A,
   'B', Foo_B,
   'C', Foo_C
    )) t2 as key_1, FooVal
    LATERAL VIEW explode (map(
    'A', Bar_A,
    'B', Bar_B,
    'C', Bar_C
     )) t3 as key_2, BarVal
     where t2.key_1 = t3.key_2;