联接总是代价高昂的，因为您必须为表1中的每个元组扫描第二个表。考虑下面的例子

A = LOAD 'data1' AS (a1:int,a2:int,a3:int);

DUMP A;
(1,2,3)
(4,2,1)
(8,3,4)
(4,3,3)
(7,2,5)
(8,4,3)

B = LOAD 'data2' AS (b1:int,b2:int);

DUMP B;
(2,4)
(8,9)
(1,3)
(2,7)
(2,9)
(4,6)
(4,9)

X = JOIN A BY a1, B BY b1;

DUMP X;
(1,2,3,1,3)
(4,2,1,4,6)
(4,3,3,4,6)
(4,2,1,4,9)
(4,3,3,4,9)
(8,3,4,8,9)
(8,4,3,8,9)

当我们加入x时，我们遍历b中的每个元组，得到a中的每个元组。对于filter，我们只遍历一次数据集，并对每个元组执行filter操作。

X = FILTER A BY a3 == 3;

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