I want to access values in maps by tuple fields.
A = load 'data' as (id:int, M:map);
B = load 'other_data' as (id:int, cat:chararray);
C = JOIN A by id, C by id;
D = foreach A generate A:id, M#cat as hash_value; /* the value in cat for each record serves as the key in the map to retrieve its value. */