Use The Index, Luke (A Guide to SQL Performance by Markus Winand) is an interesting site devoted to improving the use of B-tree indexes in relational databases.
Improving the uses of indexes in general is a good idea and given the use of relational databases to persist topic maps, it seemed appropriate to mention it here.
It would be interesting to see comparisons of the uses of B-tree and other indexing structures for a known topic map.
Personally I suspect that the amount of local memory would make more of an impact than any algorithm but that would depend on whether access to stored topic maps is being measured or merging of topic maps. That is another open research question.
If this resource helps with planning persistence of your topic maps or if you have other comments about indexing and/or this resource, please post them.