Pig Flatten 解包操作，解元组

Flatten Operator

The FLATTEN operator looks like a UDF syntactically, but it is actually an operator that changes the structure of tuples and bags in a way that a UDF cannot. Flatten un-nests tuples as well as bags. The idea is the same, but the operation and result is different
for each type of structure.

For tuples, flatten substitutes the fields of a tuple in place of the tuple. For example, consider a relation that has a tuple of the form (a, (b, c)). The expression GENERATE $0, flatten($1), will cause that tuple to become (a, b, c).

For bags, the situation becomes more complicated. When we un-nest a bag, we create new tuples. If we have a relation that is made up of tuples of the form ({(b,c),(d,e)}) and we apply GENERATE flatten($0), we end up with two tuples (b,c) and (d,e). When
we remove a level of nesting in a bag, sometimes we cause a cross product to happen. For example, consider a relation that has a tuple of the form (a, {(b,c), (d,e)}),
commonly produced by the GROUP operator. If we apply the expression GENERATE $0, flatten($1) to this tuple, we will create new tuples: (a, b, c) and (a, d, e).

Also note that the flatten of empty bag will result in that row being discarded; no output is generated. (See alsoDrop Nulls Before a Join.)

grunt> cat empty.bag
{}      1
grunt> A = LOAD 'empty.bag' AS (b : bag{}, i : int);
grunt> B = FOREACH A GENERATE flatten(b), i;
grunt> DUMP B;
grunt>

For examples using the FLATTEN operator, see FOREACH.

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