[jira] [Resolved] (SOLR-18136) ArrayStoreException in MultiThreadedSearcher when combining rerank with sort and parallel segment search

["David Smiley (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SOLR-18136?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

David Smiley resolved SOLR-18136.
---------------------------------
    Fix Version/s: 10.1
                   9.11
       Resolution: Fixed

Thanks for contributing!

> ArrayStoreException in MultiThreadedSearcher when combining rerank with sort and parallel segment search
> --------------------------------------------------------------------------------------------------------
>
>                 Key: SOLR-18136
>                 URL: https://issues.apache.org/jira/browse/SOLR-18136
>             Project: Solr
>          Issue Type: Bug
>    Affects Versions: main(11.0), 9.10.1
>            Reporter: Shiming Li
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 10.1, 9.11
>
>          Time Spent: 10m
>  Remaining Estimate: 0h
>
> *Summary*
> When multi-threaded segment-parallel search is enabled (`indexSearcherExecutorThreads > 0` and `multiThreaded=true`) 
> and a query uses both reranking (via `RankQuery` / `ReRankCollector`) and a sort, an `ArrayStoreException` is 
> thrown during the merge phase if some segments have matching documents and others do not.
>  
> *Error Message*
> {code:java}
> java.lang.ArrayStoreException: arraycopy: element type mismatch: can not cast one of the elements
> of org.apache.lucene.search.TopDocs[] to the type of the destination array,
> org.apache.lucene.search.TopFieldDocs
> {code}
>  
> *Root Cause*
> In `MultiThreadedSearcher.TopDocsCM.reduce()`, the code decides how to merge `TopDocs` from 
> different slices based solely on the type of `topDocs[0]`:
> {code:java}
> if (topDocs[0] instanceof TopFieldDocs) {
>     TopFieldDocs[] topFieldDocs = Arrays.copyOf(topDocs, topDocs.length, TopFieldDocs[].class);
>     ...
> }
> {code}
> When a ReRankCollector is used with a sort:
> Empty-result segments return TopFieldDocs (from the underlying TopFieldCollector)
> Non-empty segments return plain TopDocs (after ReRankCollector.toRescoredDocs() wraps
> results with new TopDocs(...), discarding the TopFieldDocs subtype)
> When Lucene splits segments into multiple slices for parallel search, different slices may produce
> different TopDocs subtypes. If an empty-result slice happens to be first in the array,
> topDocs[0] is TopFieldDocs, and Arrays.copyOf(..., TopFieldDocs[].class) fails because the
> array also contains plain TopDocs elements.



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