21.2. Abstract data types

One of the most well-known third generation DBMS is the Postgres system, and the creator of this system, M. Stonebreker, seems to be the inspiration for the whole direction. Many interesting tools are implemented in Postgres: a temporal storage and access model (see below) is supported, and therefore, the mechanism for logging changes, rollbacks of transactions and restoring the database after failures is completely revised; provides a powerful integrity constraint mechanism; unnormalized relations are maintained (work in this direction began in the Ingres environment), although in a rather strange way: a dynamically executed query to the database can be stored in the relation field.

One property of the Postgres system brings it closer to the properties of an object-oriented DBMS. Postgres allows data storage in fields of relations of abstract, user-defined types. This provides the possibility of introducing the behavioral aspect in the database, i.e. solves the same problem as OODB, although, of course, the semantic capabilities of the Postgres data model are significantly weaker than those of object-oriented data models. The main difference is that Postgres class systems do not assume the presence of a programming language that is equally understood by both the external programming system and the database management system. If using such a programming system, the types of data stored in the database are determined, then the DBMS is not able to control the security of these definitions, i.e. there is no guarantee that the implementation of the procedures of abstract data types will not destroy the database itself.

Note that in mid-1995, Sun Microsystems announced the release of a new product - a language and family of interpreters called Java. The Java language is an extended subset of the C ++ language. The main changes relate to the fact that the language is operatively interpreted (in the style of the BASIC language), and programs written in the Java language are guaranteed to be safe (in particular, when executing any program, the interpreter cannot be damaged). For this, in particular, arithmetic above pointers has been removed from the language. At the same time, Java remains a powerful object-oriented language, including advanced tools for defining abstract data types. Sun promotes the Java language in order to extend the capabilities of the World Wide Web Internet service (the basic idea is that from the WWW server not data is transferred to clients, but objects whose methods are programmed in Java and are interpreted on the client side; This approach, in particular, solves the problem of non-standardized representation of multimedia information). However, it seems that an interpreted and secure language such as Java can also be successfully applied to database systems that allow data to be stored with types defined by users.