@Override public boolean equals(Object o) {
      return this == o
          || o instanceof CorRef
          && uniqueKey == ((CorRef) o).uniqueKey
          && corr == ((CorRef) o).corr
          && field == ((CorRef) o).field;
    }
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