问题
Is it possible to (efficiently) select a random tuple from a bag in pig? I can just take the first result of a bag (as it is unordered), but in my case I need a proper random selection. One (not efficient) solution is counting the number of tuples in the bag, take a random number within that range, loop through the bag, and stop whenever the number of iterations matches my random number. Does anyone know of faster/better ways to do this?
回答1:
You could use RANDOM(), ORDER and LIMIT in a nested FOREACH statement to select one element with the smallest random number:
inpt = load 'group.txt' as (id:int, c1:bytearray, c2:bytearray);
groups = group inpt by id;
randoms = foreach groups {
rnds = foreach inpt generate *, RANDOM() as rnd; -- assign random number to each row in the bag
ordered_rnds = order rnds by rnd;
one_tuple = limit ordered_rnds 1; -- select tuple with the smallest random number
generate group as id, one_tuple;
};
dump randoms;
INPUT:
1 a r
1 a t
1 b r
1 b 4
1 e 4
1 h 4
1 k t
2 k k
2 j j
3 a r
3 e l
3 j l
4 a r
4 b t
4 b g
4 h b
4 j d
5 h k
OUTPUT:
(1,{(1,b,r,0.05172709255901231)})
(2,{(2,k,k,0.14351660053632986)})
(3,{(3,e,l,0.0854104195792681)})
(4,{(4,h,b,8.906013598960483E-4)})
(5,{(5,h,k,0.6219490873384448)})
If you run "dump randoms;" multiple times, you should get different results for each run.
Writing a UDF might give you better performance as you do not need to do secondary sort on random within the bag.
回答2:
I needed to do this myself, and found surprisingly that a very simple answer seems to work, to get about 10% of an alias A:
B = filter A by RANDOM() < 0.1
来源：https://stackoverflow.com/questions/14604311/selecting-random-tuple-from-bag