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Posted to issues@lucene.apache.org by "Julie Tibshirani (Jira)" <ji...@apache.org> on 2021/08/24 18:32:00 UTC

[jira] [Created] (LUCENE-10069) HNSW can miss results with very large k

Julie Tibshirani created LUCENE-10069:
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             Summary: HNSW can miss results with very large k
                 Key: LUCENE-10069
                 URL: https://issues.apache.org/jira/browse/LUCENE-10069
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Julie Tibshirani


Performing a kNN search with very large k (where k is close to the total number of live docs) can return fewer than k documents. This occurs because searches aren't guaranteed to be able to visit every node in the graph. Specifically, when choosing entry points for the search, we make k random draws _with replacement_, and sometimes end up with fewer than k entry points. These entry points may not provide a connection to all other nodes in the graph.

This is an unusual case, but I think it'd be nice if we could always return k docs when they are available (or at least document the behavior?) We're currently working on adding graph layers (LUCENE-10054), which will change how entry points are selected. Maybe we can revisit this issue once that work is further along to see if a fix is still needed.

Here's an example test failure showing the problem. We happen to select {{numDocs=101}} and {{k=100}}.
{code:java}
./gradlew test --tests TestKnnVectorQuery.testRandom -Dtests.seed=3B6CE0E105431E18
{code}



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