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:
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:
... 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 ?