Materialized views (MVs) were previously called snapshots. They are like regular views in that there is some SQL run to get the results. However unlike normal views, MVs store the results in a table. They were introduced in Oracle 8. The query used to populate the MV can go against tables, views, or even other MVs. Since the hard work is done when the MV is created, use of them can really speed up your queries.
Other databases have MVs. In Microsoft SQL Server they are called indexed views. And in DB2 they are called materialized query tables. You get the speed increase when you directly use the MVs. There is also a query rewrite technology where the database can use an MV instead of a base table to get speedup. This query rewrite was introduced in Oracle 8i.
There are different strategies to refresh the data in an MV. You can manually do it. Or you could periodically update. Or you could automatically update them when the underlying tables change. That last change gets captured in materialized view logs. There is a fast refresh option that requires specifics around the underlying query:
- No CONNECT BY
- No INTERSECT, MINUS, or UNION ALL
- No aggregate functions
- No joins other than subqueries
- No mismatch columns in a UNION
- Plus some other subquery restrictions
You cannot perform any DDL on an MV. But you can perform DML. It depends on how your MV was set up. You can make it read only. Or you could make it updatable. Do that by specifying FOR UPDATE AS prior to the SELECT. There are even writable MVs. But they are rarely used.
The query under the MV can have an ORDER BY clause. However that only applies to the data from the initial creation. Updates can make the MV out of order. The MV can be built deferred. Then the data can later be added during a refresh. Finally if you have a regular table that acts like an MV, you can turn it into an MV by using the ON PREBUILT TABLE during MV creation.