code: private static IndexedInts getRow(ColumnValueSelector s)
    {
      if (s instanceof DimensionSelector) {
        return ((DimensionSelector) s).getRow();
      } else if (s instanceof NilColumnValueSelector) {
        return ZeroIndexedInts.instance();
      } else {
        throw new ISE(
            "ColumnValueSelector[%s], only DimensionSelector or NilColumnValueSelector is supported",
            s.getClass()
        );
      }
    }
docs: Value for absent column, i. e. {@link NilColumnValueSelector}, should be equivalent to [null] during index merging.
  
  During index merging, if one of the merged indexes has absent columns, {@link StringDimensionMergerV9} ensures
  that null value is present, and it has index = 0 after sorting, because sorting puts null first. See {@link
  StringDimensionMergerV9#hasNull} and the place where it is assigned.