@Override public RelNode visit(LogicalCorrelate correlate) {
      mapCorToCorRel.put(correlate.getCorrelationId(), correlate);
      return visitJoin(correlate);
    }
Creates a CorelMap by iterating over a {@link RelNode} tree.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveRelDecorrelator.java