Schema Modiﬁcation Operators: Shneiderman and Thomas proposed in [Shneiderman and Thomas, 1982] a comprehensive set of schema changes, including structural schema changes and also changes regarding the keys and dependencies. More recently, Bernstein et al. have also proposed a set of schema evolution primitives using algebra-based constraints as their primitives [Bernstein et al., 2006, Bernstein et al., 2008]. Among several options, we chose the Schema Modiﬁcation Operators (SMOs) that we proposed in Prism and Prima. These SMOs capture the essence of the existing works, but can also express schema changes not modeled by previous approaches. For example, by using function in the ADD COLUMN operator, SMOs can support semantic conversion of columns (e.g., currency exchange), column concatenation/split (e.g., different address formats), and other similar changes that have been heavily exploited in modeling MediaWiki schema changes. The effectiveness of SMOs have been validated in [Moon et al., 2008, Curino et al., 2008c], where the Prism and Prima systems used SMOs to describe schema evolution in transaction-time databases and to support historical query reformulations over multi-schema-version transaction-time databases. The syntax of SMO is similar to that of SQL DDL ISO/IEC 9075-*: 2003, and provides a concise way to describe typical modiﬁcations of a database schema and the corresponding data migration. Every SMO takes as input a schema and produces as output a new version of the same schema. Note that simple SMOs can be arbitrarily combined in a sequence, to describe complex structural changes, as those occured in the MediaWiki DB schema evolution.
The table present the semantics of our Schema Modification Operators in terms of the effect of input and output tables and of the logical mapping defined between source and target schema versions (expressed as Disjunctive Embedded Dependencies).