Materialized view
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.[1] This is a form of caching the results of a query, similar to memoization the value of a function in functional languages, and it is sometimes described as a form of precomputation.[2][3] As with other forms of precomputation, materialized views are typically created for performance reasons, i.e. as a form of optimization.[4]
Materialized views, which store data based on remote tables, are also known as snapshots. A snapshot can be redefined as a materialized view.[clarification needed] According to C. J. Date, the term "materialized view" is deprecated in favor of "snapshot".[1]
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 This enables much more efficient access, at the cost of some data being potentially out-of-date.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.
Implementations
Oracle
Materialized views were implemented first by the Oracle Database: the Query rewrite feature was added from version 8i.[5]
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>;
PostgreSQL
In PostgreSQL, version 9.3 and newer natively support materialized views.[6] 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.[7] In version 9.4, the refresh may be concurrent with selects on the materialized view if CONCURRENTLY is used.[8]
Others
Materialized views are also supported in Sybase SQL Anywhere.[9] In IBM DB2, they are called "materialized query tables"; Microsoft SQL Server has a similar feature called "indexed views".[10] MySQL doesn't support materialized views natively, but workarounds can be implemented by using triggers or stored procedures [11] or by using the open-source application Flexviews.[12]
References
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- ↑ Oracle8i Tuning Release 8.1.5. Ecst.csuchico.edu. Retrieved on 2012-02-09.
- ↑ PostgreSQL: Materialized Views. Wiki.postgresql.org (2010-05-07). Retrieved on 2013-09-25.
- ↑ PostgreSQL: Documentation: 9.3: CREATE MATERIALIZED VIEW. PostgreSQL.com. Retrieved on 2014-01-25.
- ↑ PostgreSQL: Documentation: 9.4: REFRESHED MATERIALIZED VIEW. PostgreSQL.com. Retrieved on 2015-01-23.
- ↑ Materialized Views – Sybase SQL Anywhere. Ianywhere.com. Retrieved on 2012-02-09.
- ↑ Improving Performance with SQL Server 2005 Indexed Views. Microsoft.com. Retrieved on 2012-02-09.
- ↑ Implementing materialized views in MySQL. Shinguz.ch (2006-11-06). Retrieved on 2012-02-09.
- ↑ Flexviews for MySQL – incrementally refreshable materialized views w/ MySQL. Flexviews.sourceforge.net. Retrieved on 2012-02-09.
