@Test
  public void testInstantiateExpressionMonoParameterExpression() throws Exception {
    VectorGroupByOperator op = new VectorGroupByOperator();
    VectorAggregationDesc desc = Mockito.mock(VectorAggregationDesc.class);
    Mockito.when(desc.getVecAggrClass()).thenReturn((Class) VectorUDAFCount.class);
    VectorAggregateExpression expr = op.instantiateExpression(desc);
    Assert.assertEquals(VectorUDAFCount.class, expr.getClass());
  }
Unit test for the vectorized GROUP BY operator.
ql/src/test/org/apache/hadoop/hive/ql/exec/vector/TestVectorGroupByOperator.java