@Override
  boolean isCBOExecuted() {
    return runCBO;
  }
Checks whether Calcite can handle the query.

@param queryProperties
@param conf
@param topLevelQB
         Does QB corresponds to top most query block?
@return null if the query can be handled; non-null reason string if it
        cannot be.

        Assumption:<br>
        1. If top level QB is query then everything below it must also be
        Query<br>
        2. Nested Subquery will return false for qbToChk.getIsQuery()
ql/src/java/org/apache/hadoop/hive/ql/parse/CalcitePlanner.java