On Wed, Apr 4, 2012 at 6:15 PM, Alan Bawden <a...@basistech.com> wrote: > So I sat down to try to make a small test case that exhibited this > behavior, and while I was working on that I thought of a possible > explanation for what we are seeing. If you agree that my explanation is > what's going on here, then Benson and I can stop working on making a test > case, and move on to figuring out how we can live with what may be > unavoidable behavior. > > The key observation is that the differences in scores we see are always > down around the sixth decimal place -- down where 32-bit floating point > loses precision. So what we're seeing seems likely to simply be the result > of the fact that floating point addition isn't associative. > > In theory, the order of the documents in an index doesn't matter when > computing a score, but if the documents are stored in a different order, > any quantity that is computed using floating point by iterating over the > set of documents may come out differently due to changes in the order in > which the documents are processed. > > So could something like this cause what we are seeing?
OK this could make sense (floating point math is frustrating!). But, Lucene generally scores one document at a time, so in theory just changing its docid shouldn't alter the order of float operations. Or... is it possible the query is different? EG a BooleanQuery w/ SHOULD clauses in a different order? Maybe try explain() in the two cases and compare? Hmm, another idea: are you doing deletions in the test? Or only adds? Mike McCandless http://blog.mikemccandless.com --------------------------------------------------------------------- To unsubscribe, e-mail: java-user-unsubscr...@lucene.apache.org For additional commands, e-mail: java-user-h...@lucene.apache.org
