Release 1.3 - ...
<query> element
This element defines the facet query. Local instance vipers are used to assemble the complete query, such as the {this.searchquery()} viper, which will be substituted with the actual (master) search query.
When you need to select fields from a different table to be able to display the facet key and/or actual name, just append the necessary JOIN clause within the query element and do not use the join child-element of the facet element (which is used to specify additional JOIN clauses which must be appended within the master search query).