A materialized view provides indirect access to table data by storing the results of a query in a separate schema object. Unlike an ordinary view, which does not take up any storage space or contain any data, a materialized view contains the rows resulting from a query against one or more base tables or views. A materialized view can be stored in the same database as its base table(s) or in a different database.

Materialized views stored in the same database as their master tables can improve query performance through query rewrites. For queries that involve aggregate data or joins, the optimizer can rewrite the query to access the precomputed results stored in a materialized view. Query rewrites are particularly useful in a data warehouse environment.

Another name for materialized view is snapshot. This term generally refers to a materialized view used for replicating data in a remote database. In SQL statements, the keywords SNAPSHOT and MATERIALIZED VIEW are synonymous.

详见http://docs.oracle.com/cd/A87860_01/doc/server.817/a76965/c01intro.htm#7490