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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 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. Materialized views, which store data based on remote tables, are also known as snapshots. A snapshot can be redefined as a materialized view. According to C. J. Date, the term "materialized view" is deprecated in favor of "snapshot". 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 This enables much more efficient access, at the cost of some data being potentially out-of-date.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== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Materialized view」の詳細全文を読む スポンサード リンク
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