Extent based Sampling

• More samples obviously imply better statistics
• Instead of Tuple level sampling do page level sampling
• Important - Sampling all tuples in a random page is not the same as sampling an equal number of tuples randomly.
• However, Page level sampling is atleast as good and cost no more (in terms of I/O) than tuple level sampling. Hence it is preferred.
• Techniques to make index level sampling also fair are also available.