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https://issues.apache.org/jira/browse/LUCENE-5092?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13700141#comment-13700141
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Adrien Grand commented on LUCENE-5092:
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I think the methods to go forward and backward should be on the same instance
so I don't really like the 2nd option. The only issue I see with adding a
marker interface is that it would make filtering harder to implement, eg.
FilteredDocIdSet would need to check whether the underlying iterator supports
backwards iteration to know whether to implement {{BackwardDocIdSetIterator}}
or not.
Even if we have a marker interface, Scorers will be able to implement this
marker interface, so how is it different from adding an optional method to
DocIdSetIterator (for example I was thinking about two new methods on
DocIdSetIterator: {{canIterateBackwards()}} and {{prevDoc()}},
{{canIterateBackwards()}} returning true meaning that {{prevDoc()}} can be used
safely).
> join: don't expect all filters to be FixedBitSet instances
> ----------------------------------------------------------
>
> Key: LUCENE-5092
> URL: https://issues.apache.org/jira/browse/LUCENE-5092
> Project: Lucene - Core
> Issue Type: Improvement
> Components: modules/join
> Reporter: Adrien Grand
> Assignee: Adrien Grand
> Priority: Minor
>
> The join module throws exceptions when the parents filter isn't a
> FixedBitSet. The reason is that the join module relies on prevSetBit to find
> the first child document given a parent ID.
> As suggested by Uwe and Paul Elschot on LUCENE-5081, we could fix it by
> exposing methods in the iterators to iterate backwards. When the join modules
> gets an iterator which isn't able to iterate backwards, it would just need to
> dump its content into another DocIdSet that supports backward iteration,
> FixedBitSet for example.
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