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From "Karthick Sankarachary (JIRA)" <>
Subject [jira] Created: (LUCENE-2506) A Stateful Filter That Works Across Index Segments
Date Tue, 22 Jun 2010 01:37:57 GMT
A Stateful Filter That Works Across Index Segments

                 Key: LUCENE-2506
             Project: Lucene - Java
          Issue Type: Improvement
            Reporter: Karthick Sankarachary

By design, Lucene's Filter abstraction is applied once per segment in the index during searching.
In particular, the reader provided to its #getDocIdSet method does not represent the whole
underlying index. In other words, if the index has more than one segment the given reader
only represents a single segment. 

As a result, that definition of the Filter suffers from a limitation in that it does not have
the ability to permit/prohibit documents in the search results based on the terms residing
in not just the current segment but also the ones that came before it during the search. 

To address this limitation, we introduce here a StatefulFilter which specifically builds on
the Filter class so as to make it capable of remembering terms in segments spanning the whole
underlying index. To reiterate, the need for making filters stateful stems from the fact that
some, although not most, filters care about what terms they may have come across in prior
segments. It does so by keeping track of the past terms from prior segments in a cache that
is maintained in a StatefulTermsEnum instance on a per-thread basis.

Note that we leave it up to the concrete sub-class of the stateful filter to decide what to
remember in its state or what not to. In other words, it can choose to remember as much or
as little from prior segments as it desires. In keeping with the TermsEnum interface, which
the StatefulTermsEnum class builds on, it must let the searcher know what terms to accept
and which ones to skip over. More often than not, the state of the filter will come in handy
while implementing that very acceptance logic. 

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