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https://issues.apache.org/jira/browse/LUCENE-5409?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13879447#comment-13879447
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Peng Cheng commented on LUCENE-5409:
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I was trying to work on a test case but have encountered the following problem:
The bug will only be triggered if a ToParentBlockJoinQuery can be rewritten
into another query with a different hashcode (Uwe said this is a common
situation). But I've experimented with several simple Query and they all have
identical hashcode after the rewriting. The query that failed in my project was
a very long CustomScoreQuery (used for feature engineering in text analysis), I
wouldn't imagine to put that into the unit test. So can you show me an example
of a compound query that doesn't preserve hashcode? I can finish other works.
> ToParentBlockJoinCollector.getTopGroups returns empty Groups
> ------------------------------------------------------------
>
> Key: LUCENE-5409
> URL: https://issues.apache.org/jira/browse/LUCENE-5409
> Project: Lucene - Core
> Issue Type: Bug
> Components: core/search
> Affects Versions: 4.6
> Environment: Ubuntu 12.04
> Reporter: Peng Cheng
> Assignee: Michael McCandless
> Priority: Critical
> Fix For: 4.7
>
> Original Estimate: 168h
> Remaining Estimate: 168h
>
> A bug is observed to cause unstable results returned by the getTopGroups
> function of class ToParentBlockJoinCollector.
> In the scorer generation stage, the ToParentBlockJoinCollector will
> automatically rewrite all the associated ToParentBlockJoinQuery (and their
> subqueries), and save them into its in-memory Look-up table, namely
> joinQueryID (see enroll() method for detail). Unfortunately, in the
> getTopGroups method, the new ToParentBlockJoinQuery parameter is not
> rewritten (at least users are not expected to do so). When the new one is
> searched in the old lookup table (considering the impact of rewrite() on
> hashCode()), the lookup will largely fail and eventually end up with a
> topGroup collection consisting of only empty groups (their hitCounts are
> guaranteed to be zero).
> An easy fix would be to rewrite the original BlockJoinQuery before invoking
> getTopGroups method. However, the computational cost of this is not optimal.
> A better but slightly more complex solution would be to save unrewrited
> Queries into the lookup table.
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