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https://issues.apache.org/jira/browse/LUCENE-4607?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13527995#comment-13527995
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Robert Muir commented on LUCENE-4607:
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When i did the cost estimate patch on LUCENE-4236, i chose a long too. but
there it was trying to estimate the number of documents visited,
e.g. the number of postings.
so the formula for a conjunction would be min(subscorer cost) * #subscorers,
and for a disjunction its just the sum of all the subscorer costs, and so on.
I felt like for scoring purposes this is more useful than the number of
documents, but thats just my opinion.
> Add estimateDocCount to DocIdSetIterator
> ----------------------------------------
>
> Key: LUCENE-4607
> URL: https://issues.apache.org/jira/browse/LUCENE-4607
> Project: Lucene - Core
> Issue Type: Bug
> Components: core/search
> Affects Versions: 4.0
> Reporter: Simon Willnauer
> Fix For: 4.1, 5.0
>
> Attachments: LUCENE-4607.patch
>
>
> this is essentially a spinnoff from LUCENE-4236
> We currently have no way to make any decsision on how costly a DISI is
> neither when we apply filters nor when we build conjunctions in BQ. Yet we
> have most of the information already and can easily expose them via a cost
> API such that BS and FilteredQuery can apply optimizations on per segment
> basis.
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