Materialized View Selection for Data Warehouse Design

Background

Figure 1 shows a simplified DW architecture. The DW contains a set of materialized views. The users address their queries to the DW. The materialized views are used partially or completely for evaluating the user queries. This is achieved through partial or complete rewritings of the queries using the materialized views.

Figure 1.

A simplified DW architecture

When the source relations change, the materialized views need to be updated. The materialized views are usually maintained using an incremental strategy. With such a strategy, the changes to the source relations are propagated to the DW. The changes to the materialized views are computed using the changes of the source relations, and are eventually applied to the materialized views. The expressions used to compute the view changes involve the changes of the source relations, and are called maintenance expressions. Maintenance expressions are issued by the DW against the data sources and the answers are sent back to the DW. When the source relation changes affect more than one materialized view, multiple maintenance expressions need to be evaluated. The techniques of multiquery optimization can be used to detect ``common subexpressions’’ among maintenance expressions in order to derive an efficient global evaluation plan for all the maintenance expressions.

Main Thrust Of The Chapter

When selecting views to materialize in a DW, one attempts to satisfy one or more design goals. A design goal is either the minimization of a cost function or a constraint. A constraint can be classified as user oriented or system oriented. Attempting to satisfy the constraints can result in no feasible solution to the view selection problem. The design goals determine the design of the algorithms that select views to materialize from the space of alternative view sets.