Alan Woodward created LUCENE-7628:
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Summary: Add a getMatchingChildren() method to DisjunctionScorer
Key: LUCENE-7628
URL: https://issues.apache.org/jira/browse/LUCENE-7628
Project: Lucene - Core
Issue Type: Improvement
Reporter: Alan Woodward
Assignee: Alan Woodward
Priority: Minor
This one is a bit convoluted, so bear with me...
The luwak highlighter works by rewriting queries into their Span-equivalents,
and then running them with a special Collector. At each matching doc, the
highlighter gathers all the Spans objects positioned on the current doc and
collects their positions using the SpanCollection API.
Some queries can't be translated into Spans. For those queries that generate
Scorers with ChildScorers, like BooleanQuery, we can call .getChildren() on the
Scorer and see if any of them are SpanScorers, and for those that aren't we can
call .getChildren() again and recurse down. For each child scorer, we check
that it's positioned on the current document, so non-matching subscorers can be
skipped.
This all works correctly *except* in the case of a DisjunctionScorer where one
of the children is a two-phase iterator that has matched its approximation, but
not its refinement query. A SpanScorer in this situation will be correctly
positioned on the current document, but its Spans will be in an undefined
state, meaning the highlighter will either collect incorrect hits, or it will
throw an Exception and prevent hits being collected from other subspans.
We've tried various ways around this (including forking SpanNearQuery and
adding a bunch of slow position checks to it that are used only by the
highlighting code), but it turns out that the simplest fix is to add a new
method to DisjunctionScorer that only returns the currently matching child
Scorers. It's a bit of a hack, and it won't be used anywhere else, but it's a
fairly small and contained hack.
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