private static Set<ImmutableBitSet> getUniqueKeys(SetOp rel) {
    if (!rel.all) {
      return ImmutableSet.of(
          ImmutableBitSet.range(rel.getRowType().getFieldCount()));
    }
    return ImmutableSet.of();
  }
EstimateUniqueKeys provides an ability to estimate unique keys based on statistics.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/stats/EstimateUniqueKeys.java