What is the difference between Views and Materialized Views in Oracle?

Question

What is the difference between Views and Materialized Views in Oracle?

Answer 1

Materialized views are disk based and update periodically base upon the query definition.

Views are virtual only and run the query definition each time they are accessed.

Answer 2

A view uses a query to pull data from the underlying tables.

A materialized view is a table on disk that contains the result set of a query.

Materialized views are primarily used to increase application performance when it isn't feasible or desirable to use a standard view with indexes applied to it. The downside is that you have to use triggers, or some other automatic method, to keep the materialized view up to date.

Answer 3

Views are essentially logical table-like structures populated on the fly by a given query. The results of a view query are not stored anywhere on disk and the view is recreated every time the query is executed. Materialized views are actual structures stored within the database and written to disk. They are updated based on the parameters defined when they are created.

Answer 4

Views evaluate the data in the tables underlying the view definition at the time the view is queried. It is a logical view of your tables, with no data stored anywhere else. The upside of a view is that it will always return the latest data to you. The downside of a view is that its performance depends on how good a select statement the view is based on. If the select statement used by the view joins many tables, or uses joins based on non-indexed columns, the view could perform poorly.

Materialized views are similar to regular views, in that they are a logical view of your data (based on a select statement), however, the underlying query resultset has been saved to a table. The upside of this is that when you query a materialized view, you are querying a table, which may also be indexed. In addition, because all the joins have been resolved at materialized view refresh time, you pay the price of the join once (or as often as you refresh your materialized view), rather than each time you select from the materialized view. In addition, with query rewrite enabled, Oracle can optimize a query that selects from the source of your materialized view in such a way that it instead reads from your materialized view. In situations where you create materialized views as forms of aggregate tables, or as copies of frequently executed queries, this can greatly speed up the response time of your end user application. The downside though is that the data you get back from the materialized view is only as up to date as the last time the materialized view has been refreshed.

Materialized views can be set to refresh manually, on a set schedule, or based on the database detecting a change in data from one of the underlying tables. Materialized views can be incrementally updated by combining them with materialized view logs, which act as change data capture sources on the underlying tables.

Materialized views are most often used in data warehousing / business intelligence applications where querying large fact tables with thousands of millions of rows would result in query response times that resulted in an unusable application.

Answer 5

Adding to Mike McAllister's pretty-thorough answer...

Materialized views can only be set to refresh automatically through the database detecting changes when the view query is considered simple by the compiler. If it's considered too complex, it won't be able to set up what are essentially internal triggers to track changes in the source tables to only update the changed rows in the mview table.

When you create a materialized view, you'll find that Oracle creates both the mview and as a table with the same name, which can make things confusing.

Answer 6

A view is a logical table based on a table or other view -->Types of view -1-Simple view -2-complex view

A materialized view is a stored summary containing precomputes results (originating from an SQL select statement). -->Types of materialized views -1-Read only materialized view -2-Updateable materialized view -3-Writeable materialized view