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Shadow table
Shadow Tables are objects in computer science used to improve the way machines, networks and programs handle information.〔(【引用サイトリンク】url=http://supportingadvancement.com/records/shadow_databases/01_shadow_databases.htm )〕 More specifically, a shadow table is an object that is read and written by a processor and contains data similar to (in the same format as) its primary table, which is the table it's "shadowing". Shadow tables usually contain data that is relevant to the operation and maintenance of its primary table, but not within the subset of data required for the primary table to exist. Shadow tables are related to the data type "trails" in data storage systems. Trails are very similar to shadow tables but instead of storing identically formatted information that is different (like shadow tables), they store a history of modifications and functions operated on a table.
== History ==
Shadow tables, as an abstract concept, have been used since the beginning of modern computing. However, widespread usage of the specific phrase "shadow table" began when relational database management systems (RDBMS) became widely used in the 1970s.
The initial usage of relational DBMs for commercial purposes lead to the term "shadow tables" becoming widespread. A relational DBM uses related data fields (columns) to correlate information between tables. For example, two tables, transaction_user and transaction_amount, would both contain the column "key", and keys between tables would match, making it easy to find both the user and the amount of a specific transaction if the key is known. This relational technology allowed people to correlate information stored in a primary table and its shadow.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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