Static index pruning by in-document term frequency (Carmel pruning)
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Key: LUCENE-1812
URL: https://issues.apache.org/jira/browse/LUCENE-1812
Project: Lucene - Java
Issue Type: New Feature
Components: contrib/*
Affects Versions: 2.9
Reporter: Andrzej Bialecki
This module provides tools to produce a subset of input indexes by removing
postings data for those terms where their in-document frequency is below a
specified threshold. The net effect of this processing is a much smaller index
that for common types of queries returns nearly identical top-N results as
compared with the original index, but with increased performance.
Optionally, stored values and term vectors can also be removed. This
functionality is largely independent, so it can be used without term pruning
(when term freq. threshold is set to 1).
As the threshold value increases, the total size of the index decreases, search
performance increases, and recall decreases (i.e. search quality deteriorates).
NOTE: especially phrase recall deteriorates significantly at higher threshold
values.
Primary purpose of this class is to produce small first-tier indexes that fit
completely in RAM, and store these indexes using
IndexWriter.addIndexes(IndexReader[]). Usually the performance of this class
will not be sufficient to use the resulting index view for on-the-fly pruning
and searching.
NOTE: If the input index is optimized (i.e. doesn't contain deletions) then the
index produced via IndexWriter.addIndexes(IndexReader[]) will preserve internal
document id-s so that they are in sync with the original index. This means that
all other auxiliary information not necessary for first-tier processing, such
as some stored fields, can also be removed, to be quickly retrieved on-demand
from the original index using the same internal document id.
Threshold values can be specified globally (for terms in all fields) using
defaultThreshold parameter, and can be overriden using per-field or per-term
values supplied in a thresholds map. Keys in this map are either field names,
or terms in field:text format. The precedence of these values is the following:
first a per-term threshold is used if present, then per-field threshold if
present, and finally the default threshold.
A command-line tool (PruningTool) is provided for convenience. At this moment
it doesn't support all functionality available through API.
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