Advanced Query Optimization

Abstract

Query optimization has been an active area of research ever since the first relational systems were implemented. In the last few years, research in the area has experienced renewed impulse, thanks to new development like data warehousing. In this article, we overview some of the recent advances made in complex query optimization. This article assumes knowledge of SQL and relational algebra, as well as the basics of query processing; in particular, the user is assumed to understand cost optimization of basic SQL blocks (Select-Project-Join queries). After explaining the basic unnesting approach to provide some background, we overview three complementary techniques: source and algebraic transformations (in particular, moving outerjoins and pushing down aggregates), query rewrite (materialized views), new indexing techniques (bitmap and join indices), and different methods to build the answer (online aggregation and sampling). Throughout this article, we will use subquery to refer to a nested SQL query and outer query to refer to an SQL query that contains a nested query. The TPCH benchmark database (TPC, n.d.) is used as a source of examples. This database includes (in ascending size order) tables Nation, Customer (with a foreign key for Nation), Order (with a foreign key for Customer), and Lineitem (with a foreign key for Order). All attributes from a table are prefixed with the initial of the table’s name (“c_” for Customer, and so on).