[gmossessian]

This was a problem with the very first working implementation! However, with some intermediate data structures we were able to take advantage of the fact that, for an index with millions of items, adding or deleting a few thousand probably does not change the set of prefixes or their relative rankings very much. So although the initial precomputation of the prefix mappings from scratch takes ~10 minutes for our largest customers, we were able to get subsequent index updates down to mere seconds.