public int estimate() {
    return -1;
  }
Although similar to AbstractAggregationBuffer::estimate(), it differs from it in 2 aspects
1) This avoids creation of AggregationBuffer which may result in large memory allocation
2) This is used only while compiling query as oppose to AbstractAggregationBuffer version
which may be used in both runtime as well as compile time.
@return
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFEvaluator.java