[jira] [Created] (LUCENE-7337) MultiTermQuery are sometimes rewritten into an empty boolean query

["Ferenczi Jim (JIRA)" <ji...@apache.org>]

Ferenczi Jim created LUCENE-7337:
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             Summary: MultiTermQuery are sometimes rewritten into an empty boolean query
                 Key: LUCENE-7337
                 URL: https://issues.apache.org/jira/browse/LUCENE-7337
             Project: Lucene - Core
          Issue Type: Bug
          Components: core/search
            Reporter: Ferenczi Jim
            Priority: Minor


MultiTermQuery are sometimes rewritten to an empty boolean query (depending on the rewrite method), it can happen when no expansions are found on a fuzzy query for instance.
It can be problematic when the multi term query is boosted. 
For instance consider the following query:

`((title:bar~1)^100 text:bar)`

This is a boolean query with two optional clauses. The first one is a fuzzy query on the field title with a boost of 100. 
If there is no expansion for "title:bar~1" the query is rewritten into:

`(()^100 text:bar)`

... and when expansions are found:

`((title:bars | title:bar)^100 text:bar)`

The scoring of those two queries will differ because the normalization factor and the norm for the first query will be equal to 1 (the boost is ignored because the empty boolean query is not taken into account for the computation of the normalization factor) whereas the second query will have a normalization factor of 10,000 (100*100) and a norm equal to 0.01. 

This kind of discrepancy can happen in a single index because the expansions for the fuzzy query are done at the segment level. It can also happen when multiple indices are requested (Solr/ElasticSearch case).

A simple fix would be to replace the empty boolean query produced by the multi term query with a MatchNoDocsQuery but I am not sure that it's the best way to fix. WDYT ?
 



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