如何在猪中生成一定数量的元组？

Question

我有以下数据集：

答：

x1 y z1 
x2 y z2 
x3 y z3 
x43 y z33 
x4 y2 z4 
x5 y2 z5 
x6 y2 z6 
x7 y2 z7 

B：

y 12 
y2 25 

加载：LOAD '$输入' USING PigStorage（）AS（K： chararray，m：chararray，n：chararray）; 加载B：LOAD'$ input2'使用PigStorage（）AS（o：chararray，p：int）;

我在o上加入了m和b。我想要做的是仅为每个o选择x个元组。因此，举例来说，如果x为2它的结果是：

x1 y z1 
x2 y z2 
x4 y2 z4 
x5 y2 z5 

Answer 1

要做到这一点，你需要使用GROUP BY，FOREACH与嵌套LIMIT，比JOIN或协同组。见实施猪0.10，我用你的输入数据，以获得指定的输出：

A = load '~/pig/data/subset_join_A.dat' as (k:chararray, m:chararray, n:chararray); 
B = load '~/pig/data/subset_join_B.dat' as (o:chararray, p:int); 
-- as join will be on m, we need to leave only 2 rows per a value in m. 
group_A = group A by m; 
top_A_x = foreach group_A { 
    top = limit A 2; -- where x = 2 
    generate flatten(top); 
}; 

-- another way to do join, allows us to do left or right joins and checks 
co_join = cogroup top_A_x by (m), B by (o); 
-- filter out records from A that are not in B 
filter_join = filter co_join by IsEmpty(B) == false; 
result = foreach filter_join generate flatten(top_A_x); 

或者你可以只是一个协同组实现它，FOREACH与嵌套LIMIT：

A = load '~/pig/data/subset_join_A.dat' as (k:chararray, m:chararray, n:chararray); 
B = load '~/pig/data/subset_join_B.dat' as (o:chararray, p:int); 

co_join = cogroup A by (m), B by (o); 
filter_join = filter co_join by IsEmpty(B) == false; 
result = foreach filter_join { 
    top = limit A 2; 
--you can limit B as well 
    generate flatten(top); 
};