[Dovecot] When are search indexes updated?

I noticed that occasionally searching in my huge archive mailbox can be really slow, so I tried doveadm index on it and it seemed to do a lot of work, which seemed strange given, for example, that dovecot-lda says it keeps Dovecot index files up-to-date. Then I thought, "maybe these are different files than the search indices." If so, that's not entirely clear from the docs and Wiki. So, questions:

• When are search indexes updated?
• Are they updated incrementally?
• If not, why not?
• If so, why would a mailbox's index drift out-of-date, as mine had?

BTW, I'm using the clucene search backend.

-- Dave Abrahams BoostPro Computing Software Development Training http://www.boostpro.com Clang/LLVM/EDG Compilers C++ Boost