For densely packed IDs (32 or 64 bit integers...) I think it's even faster to use a keyed trie structure, as it can use fast bit-shifting operations in contrast to using standard B+-trees.

Also, this can be used as a persistent, on-disk / flash drive data structure or as a hash array mapped trie for HashMap implementations in-memory (Scala or Closure for instance).

For a persistent (both in the sense of functional programming as well as storing on non-volatile disk), this is for instance implemented in SirixDB (https://sirix.io), a temporal data store prototype I'm maintaining and developing :-)