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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