如何通过（b，a）过滤（a，b）关系？

Question

我有一个通用的关系的这样的：

DUMP A; 
(a, b) 
(a, c) 
(a, d) 
(b, a) 
(d, a) 
(d, b) 

看到，有一对（A，B）和（B，A）;但（d，b）没有一对。 我想过滤这些“不成对”的元组。

最终的结果应该是这样的：

DUMP R; 
(a, b) 
(a, d) 
(b, a) 
(d, a) 

我怎么可以这样写对猪？

我可以用下面的代码来解决，但交叉操作太贵：

A_cp = FOREACH L GENERATE u1, u2; 
X = CROSS A, A_cp; 
F = FILTER X BY ($0 == $3 AND $1 == $2); 
R = FOREACH F GENERATE $0, $1; 

Answer 1

这是我DESCRIBE A ; DUMP A ;的输出：

A: {first: chararray,second: chararray} 
(a,b) 
(a,c) 
(a,d) 
(b,a) 
(d,a) 
(d,b) 

这是一种方式，你可以解决这个问题：

A = LOAD 'foo.in' AS (first:chararray, second:chararray) ; 
-- Can't do a join on its self, so we have to duplicate A 
A2 = FOREACH A GENERATE * ; 

-- Join the As so that are in (b,a,a,c) etc. pairs. 
B = JOIN A BY second, A2 BY first ; 

-- We only want pairs where the first char is equal to the last char. 
C = FOREACH (FILTER B BY A::first == A2::second) 
    -- Now we project out just one side of the pair. 
    GENERATE A::first AS first, A::second AS second ; 

输出：

C: {first: chararray,second: chararray} 
(b,a) 
(d,a) 
(a,b) 
(a,d) 

更新：作为WinnieNicklaus指出，这可以缩短为：

B = FOREACH (JOIN A BY (first, second), A2 BY (second, first)) 
    GENERATE A::first AS first, A::second AS second ;