private List<HiveCorrelationInfo> getCorrelationInfos() {
    return correlationInfos;
  }
CorrelationIdVisitor digs out information from a given RexNode about all the RexSubQuery
RexNodes in the top layer. It does not go into RexSubQuery nodes within a RexSubQuery node.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/correlation/CorrelationInfoVisitor.java