The ESTCARD REST API method provides fast range counts.
Extend this with an optional boolean parameter (exact:boolean) that defaults to false for an exact range count. Note that (depending on the deployment model) exact range counts may require a scan of the appropriate statement index (this is true if using full read/write transactions and in scale-out).
Note: The historical behavior corresponds to the new defaults: exact:=false.
Note: This change is being driven by compliance with the openrdf Repository API.