[postgis-users] HELP does anybody use create index rtree ? or gist trees ?

[molinet]

Hello
I want to create an index on geometry point like
create index rtrville on ville using rtree(coord_ville);
I have the following error :
PostgreSQL error message: error: defineindex type point has no default 
operator class
Does somebody can help me ?
What are the features for rtree index and the performances ? Why do
Postgis use gist index ?
thanks
MOLINET S.