Adrien Grand created LUCENE-7421:
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Summary: Speed up BS2 by caching the 2nd lowest doc id in the
priority queue
Key: LUCENE-7421
URL: https://issues.apache.org/jira/browse/LUCENE-7421
Project: Lucene - Core
Issue Type: Improvement
Reporter: Adrien Grand
Assignee: Adrien Grand
Priority: Minor
BS2 uses a priority queue in order to merge several sorted iterators into a new
sorted iterator. We call updateTop every time that we move the 'top' iterator
forward, which requires to check the size of the priority queue at least twice
and perform at least two comparisons of doc ids.
Instead, DisjunctionSumScorer could cache the 2nd lowest doc id of the priority
queue and only call updateTop when the doc id of the entry at the top of the
priority queue goes beyong the 2nd lowest doc id. While this would involve
slightly more work in the case that the PQ has two high-cardinality clauses
whose docs are interleaved, this would help when one clause has a much higher
cardinality than the other ones or when the doc ids of the various clauses are
clustered.
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