Schema evolution deals with the need to retain current data when database schema changes are performed. Formally, Schema Evolution is accommodated when a database system facilitates database schema modification without the loss of existing data, (q.v. the stronger concept of Schema Versioning) (Schema evolution and schema versioning has been conflated in the literature with the two terms occasionally being used interchangeably. Readers are thus also encouraged to read also the entry for Schema Versioning.).
Since schemata change and/or multiple schemata are often required, there is a need to ensure that extant data either stays consistent with the revised schema or is explicitly deleted as part of the change process. A database that supports schema evolution supports this transformation process.
The first schema evolutioning proposals discussed database conversion primarily in terms of a set of transformations from one schema to another . These...
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