The "all field" has been introduced to resolve performance issues because it allows a full text query to run on 2 fields (all and all.plain) vs 14 (title, title.plain, redirect ...) when disabled.
It has been identified as problematic with some queries:
- kennedy : JFK ranked #12 with the all field and #2 with the all field disabled.
- T107666 : check this ticket for more technical details
- T116706 : disabling the all fields resolve most of the examples listed in this tickets
The problem is that the all field does not play well with the core lucene scoring functions. Cirrus tries to boost the score if the searched terms appear in the title. This boost is somewhat cancelled by the all field in certain condition (if the searched term is not very rare).
We should re-evaluate the benefits of the all field. If performance is still a concern and the all field cannot be avoided for performance reasons we could work on workarounds to compensate the title boosting issue.