1. A method for sampling data from a relational database, comprising: choosing rows from a relational database for sampling, wherein rows are arranged into pages and include column values, pages are arranged into tables and tables comprise rows and columns; wherein pages are chosen for sampling according to a probability P and rows on each selected page are chosen for sampling according to a probability R, so that an overall probability of choosing a row for sampling is Q&equals;PR; and wherein P and R are based on desired processing speed and desired precision.