A Stateful Filter That Works Across Index Segments
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Key: LUCENE-2506
URL: https://issues.apache.org/jira/browse/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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