Affects Version/s: 1.4
Fix Version/s: None
Operating System: other
When MultiSearcher invokes its subsearchers, it is the subsearchers' docFreq()
that is accessed by Similarity.docFreq(). This causes idf's to be computed
local to each index rather than globally, which causes ranking across multiple
indices to not be equivalent to ranking across the entire global collection.
The attached files (if I can figure out how to attach them) provide a potential
partial solution for this. They properly fix a simple test case, RankingTest,
that was provided by Daniel Naber.
The changes are:
1. Searcher: Add topmostSearcher() field with getter and setter to record
the outermost Searcher. Default to this.
2. MultiSearcher: Pass down the topmostSearcher when creating the subsearchers.
3. IndexSearcher: Call Query.weight() everywhere with the topmostSearcher
instead of this.
4. Query: Provide a default implementation of Query.combine() so that
MultiSearcher works with all queries.
Problems or possible problems I see:
1. This does not address the same issue with RemoteSearchable.
RemoteSearchable is not a Searcher, nor can it be due to lack of multiple
inheritance in Java, but Query.weight() requires a Searcher. Perhaps
Query.weight() should be changed to take a Searchable, but this requires
changing many places and I suspect would break apps.
2. There may be other places that topmostSearcher should be used instead of this.
3. The default implementation for Query.combine() is a guess on my part - it
works for TermQuery. It's fragile in that the default implementation will hide
bugs caused by queries that inadvertently omit a more precise Query.combine()
4. The prior comment on Query.combine() indicates that whoever wrote it was
fully aware of this problem and so probably had another usage in mind, so the
whole issue may just be Daniel's usage in the test case. It's not apparent to
me, so I probably don't understand something.