@Override public int hashCode() {
      return Objects.hash(uniqueKey, corr, 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