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

["Julie Tibshirani (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/LUCENE-10069?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Julie Tibshirani resolved LUCENE-10069.
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    Resolution: Won't Fix

> 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: Bug
>            Reporter: Julie Tibshirani
>            Priority: Minor
>          Time Spent: 20m
>  Remaining Estimate: 0h
>
> 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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