To extract unique values from a column in a relation you can use DISTINCT or GROUP BY/GENERATE. DISTINCT is the preferred method; it is faster and more efficient.
Example using GROUP BY - GENERATE:
A = load 'myfile' as (t, u, v);
B = foreach A generate u;
C = group B by u;
D = foreach C generate group as uniquekey;
dump D;
Example using DISTINCT:
A = load 'myfile' as (t, u, v);
B = foreach A generate u;
C = distinct B;
dump C;
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