title: Apache Druid - Unable to build plan for SQL query with 'UNION ALL' operation
body: <p>When running a simple SQL query containing the UNION ALL clause against Druid, the operation fails as it is unable to build the query plan.
  The official Druid documentation shows that Druid supports UNION ALL queries.</p>
  
  <p><code>SELECT column_name FROM table_1 UNION ALL SELECT column_name FROM table_2</code></p>
  
  <p>I expect a concatenated result set from the execution of both the queries.
  I am instead getting the following error - <code>{"error":"Unknown exception","errorMessage":"Cannot build plan for query: .....","errorClass":"io.druid.java.util.common.ISE","host":null}</code></p>