public Set<CorrelationId> getCorrelationIds() {
      return correlationIds;
    }
InnerRexSubQueryVisitor is a shuttle that only gets called during the RexSubQuery RexNode
visit.  The RexSubQuery has RelNodes inside. These RelNodes contain the correlationIds used
for the subquery within their RexNodes.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/correlation/CorrelationInfoVisitor.java