Trustfall's Adapter API that data providers implement leaves the joining to the adapter implementation. It provides an iterator of rows and an edge (essentially a join predicate), and asks for an iterator of (row, iterator of rows to which it is joined) back.

The adapter is free to implement whatever join algorithm makes sense, together with any predicate pushdown, caching, batching, prefetching, or any other kind of optimization.

At a high level, Trustfall's job is to (1) perform any optimizations it can do by itself, so you don't have to do them by hand, and (2) empower the adapter implementation to add any other optimizations you wish to have, without getting in the way.