Having multiple indexes complicates the maintenance of collection-wide
statistics. For example, it affects the spelling correction algorithm in Sec-tion 3.3 (page 56) that selects the corrected alternative with the most hits.
With multiple indexes and an invalidation bit vector, the correct number of
hits for a term is no longer a simple lookup. In fact, all aspects of an IR
system – index maintenance, query processing, distribution, and so on-are
more complex in logarithmic merging.