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https://issues.apache.org/jira/browse/LUCENE-5092?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13700202#comment-13700202
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Adrien Grand commented on LUCENE-5092:
--------------------------------------

bq. I dont think we should add backwards iteration to DocIDSetIterator etc. 
This is not really how search engines work, its just an oddity of join.

In that case, a simple fix would just be to dump the content into a FixedBitSet 
every time we get something else.

bq. In general I am not happy with the backwards iteration at all.

Since the actual problem looks like we require random-access on top of an API 
which only supports random-access optionally, this makes me wonder whether we 
could support the same functionality on top of NumericDocValues instead of 
filters?
                
> 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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