In computing, a materialized view is a database object that contains the results of a query. For example, it may be a local copy of data located remotely, or may be a subset of the rows and/or columns of a table or join result, or may be a summary using an aggregate function.
The process of creating a materialized view is sometimes called materialization. This is a form of caching the results of a query, similar to memoization of the value of a function in functional languages, and it is sometimes described as a form of precomputation. As with other forms of precomputation, materialized views are typically created for performance reasons, i.e. as a form of optimization.
In any database management system following the relational model, a view is a virtual table representing the result of a database query. Whenever a query or an update addresses an ordinary view's virtual table, the DBMS converts these into queries or updates against the underlying base tables. A materialized view takes a different approach in which the query result is cached as a concrete table that may be updated from the original base tables from time to time. This enables much more efficient access, at the cost of some data being potentially out-of-date. It is most useful in data warehousing scenarios, where frequent queries of the actual base tables can be expensive.
In a materialized view, indexes can be built on any column. In contrast, in a normal view, it's typically only possible to exploit indexes on columns that come directly from (or have a mapping to) indexed columns in the base tables; often this functionality is not offered at all.
Example syntax to create a materialized view in Oracle:
CREATE MATERIALIZED VIEW MV_MY_VIEW REFRESH FAST START WITH SYSDATE NEXT SYSDATE + 1 AS SELECT * FROM <table_name>;
In PostgreSQL, version 9.3 and newer natively support materialized views. In version 9.3, a materialized view is not auto-refreshed, and is populated only at time of creation (unless
WITH NO DATA is used). It may be refreshed later manually using
REFRESH MATERIALIZED VIEW. In version 9.4, the refresh may be concurrent with selects on the materialized view if
CONCURRENTLY is used.
Materialized views are also supported in Sybase SQL Anywhere. In IBM DB2, they are called "materialized query tables"; Microsoft SQL Server has a similar feature called "indexed views". MySQL doesn't support materialized views natively, but workarounds can be implemented by using triggers or stored procedures  or by using the open-source application Flexviews.
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