Schema server

The approach that has been taken by and large by commercial vendors is to include at most a few small features in the DBMS itself and then provide robust tooling that operates above the database. One feature that is fairly common across systems is transactional DDL-, CREATE, ALTER, and DROP statements can be bundled inside transactions and undone via a rollback. A consequence of this feature is that multiple versions of schemas at a time may be maintained for each table and potentially for rows within the table for concurrent access. Even though multiple versions of schema may exist internally within the engine, there is still only a single version available to the application. PostgreSQL, SQL Server, and DB2 all support this feature in Oracle, DDL statements implicitly mark a transaction boundary and run independently. 

On the other hand, there is little support to propagate schema changes to dependent schema objects, such as views, foreign keys, and indexes. When one alters a table, either the dependent objects must themselves be manually altered in some way, or the alteration must be aborted. The latter approach takes the majority of the time. For instance, SQL Server aborts any attempt to alter a column if it is part of any index, unless the alteration is within strict limits - namely, the alteration is a widening of a text or binary column. Dropped columns simply cannot participate in any index. DB2 has similar restrictions Oracle invalidates dependent objects like views so that they must be revalidated on next use and fails to execute them if they do not compile against the new schema version. 

Oracle Microsoft SQL Server IBM DB2 Altova Diff Dog XEM Kramer (2001), Su et al. (20011 Model-based approaches (X-Evolution, CoDEX, UML) Temporal XML schema... 

Microsoft SQL Server, like Oracle, supports storing a collection of homogeneous XML documents in a relation column (Pal et al. 2006). Whereas instances in an XML-typed column or table in Oracle must conform to a specific schema with a specific global element as root, an XML-typed column in SQL Server validates against any schema in a collection of schemas and allows any global element as root. One specifies an XML Schema Collection in SQL server using a DDL statement CREATE XML SCHEMA COLLECTION [ . ] sql.identifier AS Expression... 

Once a schema collection has been created, it can be assigned to be the schema for any column whose type is XML. Also, once the collection is created, there are only two operations that can be done on it - drop it or alter it by adding new constructs. The ALTER statement is the only form of schema evolution that SQL Server allows without manually dropping the schema, manually translating instances, and reestablishing the schema. The ALTER statement has only one form ... 

Mesiti M, Celle R, Sorrenti, MA, Guerrini G (2006) X-Evolution A system for XML schema evolution and document adaptation. In Proceedings of EDBT, 2006. Springer, Heidelberg Microsoft SQL Server 2008 R2 Data-Her Applications (2010) http //msdn.microsoft.com/en-us/ library/ee240739(SQL. 105).aspx... 

The BioMart data model is a simple modular schema composed of a central table, linked to its satellite tables by primary/foreign key relations. The schema can be normalized but typically includes denormalizations to achieve maximum query response optimizations. All the BioMart metadata is stored in XML configuration files on the database servers. The metadata files can be readily created and modified using MartEditor, a Java-based configuration editor. 

Because of the flexibility and familiarity of Web browsers, Web-based applications have become very popular and powerful. There are many Web-based interfaces to RDBMS. For example, phpPgAdmin1 is a popular Web application for PostgreSQL. PhpMyAdmin2 is popular with users of MySQL. These applications allow one to connect to a chosen database server, browse the schemas and tables in the database, and enter SQL commands. The output from SQL commands, especially table output, is formatted nicely. Other operations, such as creating, altering, or dropping tables and schemas, are also provided. These use an HTML form interface, with text boxes, radio buttons, check boxes, and other form elements familiar to all uses of the Web. 

In the Jaakko Poyry MODE concept the process application is integrated with a 3D plant design system. All information between the systems is transferred in an application-independent XML format. Using the application server in integration gives possibilities to share design work in different locations within the company or even between different companies. Currently Poyry s own legacy XML schema is used, but when standard schemas are published, those should be used. 

---