[indexing] 2d spatial + 1d value in one index?

Dear list members,

Let me first introduce myself, I'm Martijn and currently doing my master's
thesis research at Delft University (The Netherlands). I am working in the
field of spatial databases and I am going to test, investigate and
implement a data structure which is suitable for on-the-fly generalization
of map data.

I am a proponent of open source software, therefore I'd like to use
PostgreSQL. Another candidate for my research is Oracle. However, I cannot
find any information on PostreSQL if it is capable of creating a spatial
index on 2 dimensional spatial data (on its native geometry types) plus an
extra value, so in the tables there will be something like:

geometry | importance value
-------------------------------------------
line(x1, y1,...,xn, yn) | 5.0
line(x2, y2,...,xn, yn) | 2.0

Is there an (R-tree-like, i.e. reactive) index structure available which
can handle such tuples, so the query optimizer can handle a. geometry
(location in 2D) and b. the other value in one go?

Thanks in advance,

Martijn Meijers
--
MSc student at Delft University of Technology
URL: http://www.gdmc.nl/martijn/