/**
* Derives the list of column names suitable for NATURAL JOIN. These are the columns that occur
* exactly once on each side of the join.
*
* @param leftRowType Row type of left input to the join
* @param rightRowType Row type of right input to the join
* @return List of columns that occur once on each side
*/
public static List<String> deriveNaturalJoinColumnList(
RelDataType leftRowType, RelDataType rightRowType) {
List<String> naturalColumnNames = new ArrayList<String>();
final List<String> leftNames = leftRowType.getFieldNames();
final List<String> rightNames = rightRowType.getFieldNames();
for (String name : leftNames) {
if ((Collections.frequency(leftNames, name) == 1)
&& (Collections.frequency(rightNames, name) == 1)) {
naturalColumnNames.add(name);
}
}
return naturalColumnNames;
}
