Join 如何在pig中生成一定数量的元组？

我有以下数据集：

A:

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

y 12
y2 25

B:

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

y 12
y2 25

加载A：使用PigStorage（）加载“$input”，作为（k:chararray，m:chararray，n:chararray）； 加载B：使用PigStorage（）作为（o:chararray，p:int）加载“$input2”

我在m上加入A，在o上加入B。我想做的是为每个o只选择x个元组。例如，如果x为2，结果是：

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

---

为此，需要使用GROUP BY、FOREACH和嵌套限制，然后使用JOIN或COGROUP。参见Pig 0.10中的实现，我使用您输入的数据来获得指定的输出：

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，您可以只使用一个COGROUP实现它，FOREACH具有嵌套限制：

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);
};

---

为此，需要使用GROUP BY、FOREACH和嵌套限制，然后使用JOIN或COGROUP。参见Pig 0.10中的实现，我使用您输入的数据来获得指定的输出：

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，您可以只使用一个COGROUP实现它，FOREACH具有嵌套限制：

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);
};