/** * 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; }