If you are looking for an open source technology to meet this need, Apache Drill is a distributed SQL engine that can talk to multiple datasources and make them available in a single namespace. Joins across sources are supported and can actually perform well when you use enough nodes for your data volume.
Because Drill exposes ODBC and JDBC interfaces, it can very easily be used with BI tools to expose all kinds of data to analysts.
The architecture is fully pluggable so anyone can write a connector to a new datastore and just include it on the classpath when starting the cluster. In the current 1.2 release Drill can connect to any database that exposes JDBC, like MySQL, Postgres, Oracle, as well as a number of non-relational datastores like HDFS, MongoDB and HBase.