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https://issues.apache.org/jira/browse/LUCENE-10147?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17424414#comment-17424414
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Michael Sokolov commented on LUCENE-10147:
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Oh, nice catch! I agree let's shift (and maybe divide by 2?). Perhaps we should
consider adding `VectorSimilarityFunction.normalizeScore` so this normalization
can be done in `Lucene90HnswVectorsReader.search` when we populate TopDocs, and
we avoid needing to normalize every score during graph search (when it is not
needed). WDYT?
Also +1 for improving docs around normalizing the input. We had a discussion
about this and whether we should enforce, but it seemed too costly. I think
clear docs should help though.
> KnnVectorQuery can produce negative scores
> ------------------------------------------
>
> Key: LUCENE-10147
> URL: https://issues.apache.org/jira/browse/LUCENE-10147
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Julie Tibshirani
> Priority: Major
>
> The cosine similarity of two vectors falls in the range [-1, 1]. So currently
> with cosine similarity, {{KnnVectorQuery}} can produce negative scores. Maybe
> we should just adjust the scores in this case by adding 1, shifting them to
> the range [0, 2].
> As a side note, this made me notice that
> {{VectorSimilarityFunction.DOT_PRODUCT}} is really quite "expert"! Users need
> to know to normalize all document and query vectors to unit length when using
> this similarity. Otherwise the output is unbounded and difficult to handle in
> scoring. Also dot product is not a true metric: for example, it doesn't obey
> the triangle inequality. So many ANN algorithms have trouble supporting it.
> As part of this issue, we could improve the documentation on
> {{VectorSimilarityFunction.DOT_PRODUCT}} to clarify that normalization is
> required.
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