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https://issues.apache.org/jira/browse/LUCENE-6425?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14496158#comment-14496158
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Adrien Grand commented on LUCENE-6425:
--------------------------------------
bq. It might still make sense to have an extractTerms on query to get explicit
terms (those that do not depend any specific index?)
In that case, I think it would be as easy to rewrite against an empty index?
This is what PostingsHighlighter does in order to avoid costly rewrites:
{code}
IndexReader emptyReader = new MultiReader();
Set<Term> termSet = new HashSet<>();
new IndexSearcher(emptyReader).createNormalizedWeight(query,
false).extractTerms(termSet);
{code}
> Move extractTerms to Weight
> ---------------------------
>
> Key: LUCENE-6425
> URL: https://issues.apache.org/jira/browse/LUCENE-6425
> Project: Lucene - Core
> Issue Type: Task
> Reporter: Adrien Grand
> Assignee: Adrien Grand
> Priority: Minor
> Attachments: LUCENE-6425.patch
>
>
> Today we have extractTerms on Query, but it is supposed to only be called
> after the query has been specialized to a given IndexReader using
> Query.rewrite(IndexReader) to allow some complex queries to replace terms
> "matchers" with actual terms (eg. WildcardQuery).
> However, we already have an abstraction for indexreader-specialized queries:
> Weight. So I think it would make more sense to have extractTerms on Weight.
> This would also remove the trap of calling extractTerms on a query which is
> not rewritten yet.
> Since Weights know about whether scores are needed or not, I also hope this
> would help improve the extractTerms semantics. We currently have 2 use-cases
> for extractTerms: distributed IDF and highlighting. While the former only
> cares about terms which are used for scoring, it could make sense to
> highlight terms that were used for matching, even if they did not contribute
> to the score (eg. if wrapped in a ConstantScoreQuery or a BooleanQuery FILTER
> clause). So highlighters could do searcher.createNormalizedWeight(query,
> false).extractTerms(termSet) to get all terms that were used for matching the
> query while distributed IDF would instead do
> searcher.createNormalizedWeight(query, true).extractTerms(termSet) to get
> scoring terms only.
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