Enable MultiTermQuery's constant score mode to also use BooleanQuery under the
hood
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Key: LUCENE-1644
URL: https://issues.apache.org/jira/browse/LUCENE-1644
Project: Lucene - Java
Issue Type: Improvement
Components: Search
Reporter: Michael McCandless
Priority: Minor
Fix For: 2.9
When MultiTermQuery is used (via one of its subclasses, eg
WildcardQuery, PrefixQuery, FuzzyQuery, etc.), you can ask it to use
"constant score mode", which pre-builds a filter and then wraps that
filter as a ConstantScoreQuery.
If you don't set that, it instead builds a [potentially massive]
BooleanQuery with one SHOULD clause per term.
There are some limitations of this approach:
* The scores returned by the BooleanQuery are often quite
meaningless to the app, so, one should be able to use a
BooleanQuery yet get constant scores back. (Though I vaguely
remember at least one example someone raised where the scores were
useful...).
* The resulting BooleanQuery can easily have too many clauses,
throwing an extremely confusing exception to newish users.
* It'd be better to have the freedom to pick "build filter up front"
vs "build massive BooleanQuery", when constant scoring is enabled,
because they have different performance tradeoffs.
* In constant score mode, an OpenBitSet is always used, yet for
sparse bit sets this does not give good performance.
I think we could address these issues by giving BooleanQuery a
constant score mode, then empower MultiTermQuery (when in constant
score mode) to pick & choose whether to use BooleanQuery vs up-front
filter, and finally empower MultiTermQuery to pick the best (sparse vs
dense) bit set impl.
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