It would be interesting to compare in-memory use of the [R-Tree Extension](http://gist.cs.berkeley.edu/libgist-2.0/libgist_user_manual.html#usingrtext) implemented in [libgist2](http://gist.cs.berkeley.edu/libgist-2.0/) from the [GiST](http://gist.cs.berkeley.edu) project.