One technique employed in data warehouses to improve performance is the creation of summaries. Summaries are special types of aggregate views that improve query execution times by precalculating expensive joins and aggregation operations before execution and storing the results in a table in the database. For example, you can create a summary table to contain the sums of sales by region and by product.
The summaries or aggregates that are referred to in this guide and in literature on data warehousing are created in Oracle Database using a schema object called a materialized view. Materialized views in a data warehouse speed up query performance.
The database supports transparent rewrites against materialized views, so that you do not need to modify the original queries to take advantage of precalculated results in materialized views. Instead of executing the query, the database retrieves precalculated results from one or more materialized views, performs any necessary additional operations on the data, and returns the query results.