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https://issues.apache.org/jira/browse/LUCENE-8829?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16860649#comment-16860649
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Adrien Grand commented on LUCENE-8829:
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bq. However, if setShardIndex = false, then that also implies that user does
not care about shard index orderings, so we should really be using docIDs to
resolve ties then.
Actually the contract today is that TopDocs#merge should reuse the shardIndex
of the ScoreDoc objects. This might be useful when merging hits incrementally,
see e.g. LUCENE-7707 for more background. However I agree with you that passing
setShardIndex=false from IndexSearcher makes more sense since all hits
conceptually belong to the same shard. I wonder whether we could rely on the
fact that ScoreDoc objects have -1 as a shard index by default so that the
comparison always produces a tie and that we then move to comparing by doc ID.
> TopDocs#Merge is Tightly Coupled To Number Of Collectors Involved
> -----------------------------------------------------------------
>
> Key: LUCENE-8829
> URL: https://issues.apache.org/jira/browse/LUCENE-8829
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Atri Sharma
> Priority: Major
> Attachments: LUCENE-8829.patch, LUCENE-8829.patch, LUCENE-8829.patch
>
>
> While investigating LUCENE-8819, I understood that TopDocs#merge's order of
> results are indirectly dependent on the number of collectors involved in the
> merge. This is troubling because 1) The number of collectors involved in a
> merge are cost based and directly dependent on the number of slices created
> for the parallel searcher case. 2) TopN hits code path will invoke merge with
> a single Collector, so essentially, doing the same TopN query with single
> threaded and parallel threaded searcher will invoke different order of
> results, which is a bad invariant that breaks.
>
> The reason why this happens is because of the subtle way TopDocs#merge sets
> shardIndex in the ScoreDoc population during populating the priority queue
> used for merging. ShardIndex is essentially set to the ordinal of the
> collector which generates the hit. This means that the shardIndex is
> dependent on the number of collectors, even for the same set of hits.
>
> In case of no sort order specified, shardIndex is used for tie breaking when
> scores are equal. This translates to different orders for same hits with
> different shardIndices.
>
> I propose that we remove shardIndex from the default tie breaking mechanism
> and replace it with docID. DocID order is the de facto that is expected
> during collection, so it might make sense to use the same factor during tie
> breaking when scores are the same.
>
> CC: [~ivera]
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