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Posted to dev@lucene.apache.org by "Paul Elschot (JIRA)" <ji...@apache.org> on 2015/11/19 20:16:11 UTC
[jira] [Resolved] (LUCENE-6894) Improve DISI.cost() by assuming
independence for match probabilities
[ https://issues.apache.org/jira/browse/LUCENE-6894?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Paul Elschot resolved LUCENE-6894.
----------------------------------
Resolution: Not A Problem
Lucene Fields: New,Patch Available (was: New)
> Improve DISI.cost() by assuming independence for match probabilities
> --------------------------------------------------------------------
>
> Key: LUCENE-6894
> URL: https://issues.apache.org/jira/browse/LUCENE-6894
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/search
> Reporter: Paul Elschot
> Priority: Minor
> Attachments: LUCENE-6894.patch, LUCENE-6894.patch
>
>
> The DocIdSetIterator.cost() method returns an estimation of the number of matching docs. Currently conjunctions use the minimum cost, and disjunctions use the sum of the costs, and both are too high.
> The probability of a match is estimated by dividing available cost() by the number of docs in a segment.
> The conjunction probability is then the product of the inputs, and the disjunction probability follows from De Morgan's rule:
> "not (A and B)" is the same as "(not A) or (not B)"
> with the probability for "not" computed as 1 minus the input probability.
> The independence that is assumed is normally not there. However, the cost() results are only used to order the input DISIs/Scorers for optimization, and for that I expect this assumption to work nicely.
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