@Override
      public Object writeValue(ColumnVector column, int row)
          throws HiveException {
        final MapColumnVector mapColVector = (MapColumnVector) column;
        final SettableMapObjectInspector mapOI =
            (SettableMapObjectInspector) this.objectInspector;
        final Map<Object, Object> value =
            (Map<Object, Object>) vectorExtractRow.extractRowColumn(
                mapColVector, mapTypeInfo, mapOI, row);
        if (value == null) {
          return null;
        }

        mapOI.clear(obj);
        for (Map.Entry<Object, Object> entry : value.entrySet()) {
          mapOI.put(obj, entry.getKey(), entry.getValue());
        }
        return obj;
      }
Compiles the appropriate vector expression writers based on a struct object
inspector.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/VectorExpressionWriterFactory.java