private int[] getByteColumnVectorColumns(int[] outputColumns, TypeInfo[] typeInfos) {

    // Search mapping for any strings and return their output columns.
    ArrayList<Integer> list = new ArrayList<Integer>();
    final int count = outputColumns.length;
    for (int i = 0; i < count; i++) {
      int outputColumn = outputColumns[i];
      String typeName = typeInfos[i].getTypeName();
      if (VectorizationContext.isStringFamily(typeName)) {
        list.add(outputColumn);
      }
    }
    return ArrayUtils.toPrimitive(list.toArray(new Integer[0]));
  }
Determine from a mapping which columns are BytesColumnVector columns.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/mapjoin/VectorMapJoinCommonOperator.java