Hi Tom, computeNorm(FieldInvertState) method is the only place where similarity is tied to indexing process. If you want to switch between different similarities, they should share the same implementation for the method. For example, subclasses of SimilarityBase can be used without re-indexing.
By the way DefaultSimilarity and BM25 looks compatible. For memory consumption reasons, exact value of the field length is encoded/decoded into byte in norms at the expense of some precision loss. Ahmet On Wednesday, August 19, 2015 7:40 PM, Tom Burton-West <tburt...@umich.edu> wrote: Hello all, The last time I worked with changing Simlarities was with Solr 4.1 and at that time, it was possible to simply change the schema to specify the use of a different Similarity without re-indexing. This allowed me to experiment with several different ranking algorithms without having to re-index. Currently the documentation states that while doing this is theoretically possible but not well defined: "To change Similarity <http://lucene.apache.org/core/5_2_0/core/org/apache/lucene/search/similarities/Similarity.html>, one must do so for both indexing and searching, and the changes must happen before either of these actions take place. Although in theory there is nothing stopping you from changing mid-stream, it just isn't well-defined what is going to happen." http://lucene.apache.org/core/5_2_0/core/org/apache/lucene/search/similarities/package-summary.html#changingSimilarity Has something changed between 4.1 and 5.2 that actually will prevent changing Similarity without re-indexing from working, or is this just a warning in case at some future point someone contributes code so that a particular similarity takes advantage of a different index format? Tom
