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From "Adrien Grand (JIRA)" <>
Subject [jira] [Created] (LUCENE-6077) Add a filter cache
Date Wed, 26 Nov 2014 13:55:12 GMT
Adrien Grand created LUCENE-6077:

             Summary: Add a filter cache
                 Key: LUCENE-6077
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Adrien Grand
            Assignee: Adrien Grand
            Priority: Minor
             Fix For: 5.0

Lucene already has filter caching abilities through CachingWrapperFilter, but CachingWrapperFilter
requires you to know which filters you want to cache up-front.

Caching filters is not trivial. If you cache too aggressively, then you slow things down since
you need to iterate over all documents that match the filter in order to load it into an in-memory
cacheable DocIdSet. On the other hand, if you don't cache at all, you are potentially missing
interesting speed-ups on frequently-used filters.

Something that would be nice would be to have a generic filter cache that would track usage
for individual filters and make the decision to cache or not a filter on a given segments
based on usage statistics and various heuristics, such as:
 - the overhead to cache the filter (for instance some filters produce DocIdSets that are
already cacheable)
 - the cost to build the DocIdSet (the getDocIdSet method is very expensive on some filters
such as MultiTermQueryWrapperFilter that potentially need to merge lots of postings lists)
 - the segment we are searching on (flush segments will likely be merged right away so it's
probably not worth building a cache on such segments)

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