add comments from Doug describing how BooleanScorers work

git-svn-id: https://svn.apache.org/repos/asf/lucene/java/trunk@727475 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
Michael McCandless 2008-12-17 19:16:10 +00:00
parent d3987d9ed4
commit 4561cb369d
2 changed files with 36 additions and 0 deletions

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@ -19,6 +19,39 @@ package org.apache.lucene.search;
import java.io.IOException;
/* Description from Doug Cutting (excerpted from
* LUCENE-1483):
*
* BooleanScorer uses a ~16k array to score windows of
* docs. So it scores docs 0-16k first, then docs 16-32k,
* etc. For each window it iterates through all query terms
* and accumulates a score in table[doc%16k]. It also stores
* in the table a bitmask representing which terms
* contributed to the score. Non-zero scores are chained in
* a linked list. At the end of scoring each window it then
* iterates through the linked list and, if the bitmask
* matches the boolean constraints, collects a hit. For
* boolean queries with lots of frequent terms this can be
* much faster, since it does not need to update a priority
* queue for each posting, instead performing constant-time
* operations per posting. The only downside is that it
* results in hits being delivered out-of-order within the
* window, which means it cannot be nested within other
* scorers. But it works well as a top-level scorer.
*
* The new BooleanScorer2 implementation instead works by
* merging priority queues of postings, albeit with some
* clever tricks. For example, a pure conjunction (all terms
* required) does not require a priority queue. Instead it
* sorts the posting streams at the start, then repeatedly
* skips the first to to the last. If the first ever equals
* the last, then there's a hit. When some terms are
* required and some terms are optional, the conjunction can
* be evaluated first, then the optional terms can all skip
* to the match and be added to the score. Thus the
* conjunction can reduce the number of priority queue
* updates for the optional terms. */
final class BooleanScorer extends Scorer {
private SubScorer scorers = null;
private BucketTable bucketTable = new BucketTable();

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@ -22,6 +22,9 @@ import java.util.ArrayList;
import java.util.List;
import java.util.Iterator;
/* See the description in BooleanScorer.java, comparing
* BooleanScorer & BooleanScorer2 */
/** An alternative to BooleanScorer that also allows a minimum number
* of optional scorers that should match.
* <br>Implements skipTo(), and has no limitations on the numbers of added scorers.