@Override
    public int compareTo(@Nonnull CorRef o) {
      int c = corr.compareTo(o.corr);
      if (c != 0) {
        return c;
      }
      c = Integer.compare(field, o.field);
      if (c != 0) {
        return c;
      }
      return Integer.compare(uniqueKey, o.uniqueKey);
    }
A unique reference to a correlation field.

<p>For instance, if a RelNode references emp.name multiple times, it would
result in multiple {@code CorRef} objects that differ just in
{@link CorRef#uniqueKey}.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveRelDecorrelator.java