Let's see if I'm following your reasoning correctly.
I'm essentially interested in three variables:
Are you suggesting that I make a multi-dimensional Judy array of arrays? So for each movie create a Judy array using $userid as the index and $rating as the value, then put that into a Judy array as the value with $movieid as the index?
Apologies if I'm stating the obvious, I wouldn't classify myself as a programmer.
From a very, very rough test (not even gone back to confirm availability of data) this is looking very good indeed for memory consumption. Will do some further testing tomorrow