equality indexing

["Howard Chu" <hyc@highlandsun.com>]

If we used a Btree for the attribute indexes, and used the actual attribute
value (instead of the hash that we use now) for the key, we could use BDB's
Range feature to do >= indexing as well as equality. (I'm not sure we get <=
indexing out of this, haven't thought it all the way through yet.)

As a compromise, we can use a "prefix" with the hash appended. E.g., use the
first 4 bytes of the actual attribute value, plus the current equality hash.
That way our equality lookups are still reasonably compact, and still
unambiguous, but we also have a means for doing loose ordering indexing.

  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc
  Symas: Premier OpenSource Development and Support