I've implemented subj algorithm as PostgreSQL extension.
It looks like this realization is such as fast as R-Tree but sufficiently
more simple & compact.
Here (https://habrahabr.ru/post/319810/) or
you can find the algirithm description and some benchmarks in comparison
with GiST R-Tree.
The extension sources are uploaded at
under the BSD license.
Feel free to connect with me if you have some questions.