Currently a boolean predicate without any clause will not match any document with the Lucene backend, or match all documents with the Elasticsearch backend.

This behavior doesn’t seem to be documented in Elasticsearch: . Technically, you could say a boolean query without any clause is illegal in Elasticsearch, since the documentation states “It is built using one or more boolean clauses”.

We should make the behavior consistent: let’s not match any document when no clauses are provided.