hadoop - 组运算符和组与组

Question

在http://pig.apache.org/docs/r0.7.0/piglatin_ref2.html#FOREACH的以下两个示例中，group是做什么的:

Example: Nested Projection

In this example if one of the fields in the input relation is a tuple, bag or map, we can perform a projection on that field (using a deference operator).

X = FOREACH C GENERATE group, B.b2;

DUMP X; (1,{(3)}) (4,{(6),(9)}) (8,{(9)})

In this example multiple nested columns are retained.

X = FOREACH C GENERATE group, A.(a1, a2);

DUMP X; (1,{(1,2)}) (4,{(4,2),(4,3)}) (8,{(8,3),(8,4)})

使用group和使用GROUP有什么区别吗？

例子:

Example: Flattening

In this example the FLATTEN operator is used to eliminate nesting.

X = FOREACH C GENERATE group, FLATTEN(A);

DUMP X; (1,1,2,3) (4,4,2,1) (4,4,3,3) (8,8,3,4) (8,8,4,3)

Another FLATTEN example.

X = FOREACH C GENERATE GROUP, FLATTEN(A.a3);

DUMP X; (1,3) (4,1) (4,3) (8,4) (8,3)

Answer 1

从http://pig.apache.org/docs/r0.11.0/basic.html#GROUP

The first field is named "group" (do not confuse this with the GROUP operator) and is the same type as the group key.

字段名称区分大小写，因此GROUP运算符不会创建字段“GROUP”。我认为这是 pig 文档中的一个小错误。