When using Xapian for full text search, various problems show:
- only few hits are found
  indexer_xapian.py uses:
    matches = enquire.get_mset(0, 10)
  so at most 10 results are found.
  Additionally when looking at the Xapian API docs, I think the
  "checkatleast" parameter should be used:

  | the minimum number of items to check. Because the matcher optimises,
  | it won't consider every document which might match, so the total
  | number of matches is estimated. Setting checkatleast forces it to
  | consider at least this many matches and so allows for reliable paging
  | links.

  I read this as: search is unreliable if checkatleast is too low.

- it seems as if issues titles are not searched
  (though maybe this is just a symptom of above)

http://tracker.xemacs.org/XEmacs/its/issue501 describes this problem,
too, but I noticed it in our own installation.