The Aqua approximate query answering system
Swarup Acharya, Phillip B. Gibbons, Viswanath Poosala, Sridhar Ramaswamy · ACM SIGMOD Record · 1999
Aqua is a system for providing fast, approximate answers to aggregate queries, which are very common in OLAP applications. It has been designed to run on top of any commercial relational DBMS. Aqua precomputes synopses (special statistical summaries) of the original data and stores them in the DBMS. It provides approximate answers along with quality guarantees by rewriting the queries to run on these synopses. Finally, Aqua keeps the synopses up-to-date as the database changes, using fast incremental maintenance techniques.