Approximate Query Processing Framework Based on Spark

There are three advantages in this project:
1. Provide early results with error estimation.
2. Accept almost all kinds of queries.
3. Sometimes, it's a good progress estimator.

From acdemic perspecitve: 
1. Break blocks between two phases. 
2. Bulid Histogram when adding tables into cloud storage, such as HDFS. 
3. Use Online Sampling to help create a presentive subset. 
4. Show Flexible and Efficient to users.