14 March 2019, Apache Lucene™ 8.0.0 available

The Lucene PMC is pleased to announce the release of Apache Lucene 8.0.0.

Apache Lucene is a high-performance, full-featured text search engine library written entirely in Java. It is a technology suitable for nearly any application that requires full-text search, especially cross-platform.

This release contains numerous bug fixes, optimizations, and improvements, some of which are highlighted below. The release is available for immediate download at:

http://lucene.apache.org/core/mirrors-core-latest-redir.html

Lucene 8.0.0 Release Highlights:

Query execution

Term queries, phrase queries and boolean queries introduced new optimization that enables efficient skipping over non-competitive documents when the total hit count is not needed. Depending on the exact query and data distribution, queries might run between a few percents slower and many times faster, especially term queries and pure disjunctions.

In order to support this enhancement, some API changes have been made:

• TopDocs.totalHits is no longer a long but an object that gives a lower bound of the actual hit count.
• IndexSearcher's search and searchAfter methods now only compute total hit counts accurately up to 1,000 in order to enable this optimization by default.
• Queries are now required to produce non-negative scores.
  

Codecs

• Postings now index score impacts alongside skip data. This is how term queries optimize collection of top hits when hit counts are not needed.
• Doc values introduced jump tables, so that advancing runs in constant time. This is especially helpful on sparse fields.
• The terms index FST is now loaded off-heap for non-primary-key fields using MMapDirectory, reducing heap usage for such fields.
  

Custom scoring

The new FeatureField allows efficient integration of static features such as a pagerank into the score. Furthermore, the new LongPoint#newDistanceFeatureQuery and LatLonPoint#newDistanceFeatureQuery methods allow boosting by recency and geo-distance respectively. These new helpers are optimized for the case when total hit counts are not needed. For instance if the pagerank has a significant weight in your scores, then Lucene might be able to skip over documents that have a low pagerank value.

Further details of changes are available in the change log available at: http://lucene.apache.org/core/8_0_0/changes/Changes.html

Please report any feedback to the mailing lists (http://lucene.apache.org/core/discussion.html)

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