Your comment is a little bit perplexing to me because I don't really recognize much difference between composing relational operations and map / reduce operations. SQL `select` is map, `group by` is reduce, `where` is a flatMap to zero or one items, and `join` is a kind of specialized flatMap to zero or more items. You have to consider IO quantity and throughput (both disk and memory) to get decent performance either way.

For example, if you're writing straight-up SQL, the fewer columns you project out before you start sorting the better off you are, it's better to join stuff after you've done your sort than the reverse because it means physically less data shuffling.