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https://issues.apache.org/jira/browse/LUCENE-6276?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14952926#comment-14952926
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Adrien Grand commented on LUCENE-6276:
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I think it would make more sense to sum up {{totalTermFreq/docFreq}} for each
term instead of {{totalTermFreq/conjunctionDISI.cost()}}, so that we get the
average number of positions per document? But otherwise I think you got the
intention right. Something else to be careful with is that
{{TermStatistics.totalTermFreq()}} may return -1, so we need a fallback for
that case. Maybe we could just assume 1 position per document?
A related question is what definition we should give to {{matchCost()}}. The
patch does not have the issue yet since it only deals with phrase queries, but
eventually we should be able to compare the cost of eg. a phrase query against
a doc values range query even though they perform very different computations.
Maybe the javadocs of matchCost could suggest a scale of costs of operations
that implementors of matchCost() could use in order to compute the cost of
matching the two-phase iterator. It could be something like 1 for nextDoc(),
nextPosition(), comparisons and basic arithmetic operations and eg. 10 for
advance()?
> Add matchCost() api to TwoPhaseDocIdSetIterator
> -----------------------------------------------
>
> Key: LUCENE-6276
> URL: https://issues.apache.org/jira/browse/LUCENE-6276
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Robert Muir
> Attachments: LUCENE-6276-ExactPhraseOnly.patch
>
>
> We could add a method like TwoPhaseDISI.matchCost() defined as something like
> estimate of nanoseconds or similar.
> ConjunctionScorer could use this method to sort its 'twoPhaseIterators' array
> so that cheaper ones are called first. Today it has no idea if one scorer is
> a simple phrase scorer on a short field vs another that might do some geo
> calculation or more expensive stuff.
> PhraseScorers could implement this based on index statistics (e.g.
> totalTermFreq/maxDoc)
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