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Effective way to cross two relations
I have a relation of about 50000 tuples that I want to join to itself either by using a cross operator or something similar. Then I would be doing pair wise computation of half the matrix (avoiding comparing to self and duplicate).

I was wondering what the most effective way to do this,  below is some pseudo pig latin.
-- About 50,000 - 70,000 entries
a = LOAD 'part-r-00000.txt' USING PigStorage()
AS (id:long,  x:int, y:int);
-- Same as a , About 50,000 - 70,000 entries
b = LOAD 'part-r-00000.txt' USING PigStorage()
AS (id:long,  x:int, y:int);

jnd = join a by id , b by id;
-- filter comparisons to self and duplicates from the matrix
-- end up with 50000 X (50000-1)/2 entries
filter_self = filter jnd by a::id != b::id and a::id > b::id;

raw = foreach filter_self generate a::id as id1, b::id as id2, TOBAG(a::x, b::y) as z;
-- group pairs for comparison
grpd = group raw by (id1, id2);
-- calculate similarity between id1 and id2 based on a udf
prjctd = foreach grpd generate flatten(group), UDF(raw.z);

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Sergey Goder 2013-04-19, 21:13