and numbers. Storage issues can become problematic. (Web hosting domain) Relational
and numbers. Storage issues can become problematic. Relational databases use many different types of underlying disk storage techniques to make the management of records in tables more efficient. A typical record in a table may occupy at most 2 KB (sometimes known as a page or block), and often much less. Even the smallest of graphic objects used in Web site applications easily exceeds the size of a record and each record in a table could have a unique graphic object. Therefore, storing a graphic object with each record in the underlying operating system block structure completely ruins any kind of specialized storage structure performance tuned for simple table record strings and numbers storage. Binary objects were created to physically separate binary values from traditional table record values. The obvious extension to this concept was creation of binary objects to store anything in binary format, reducing storage, even items such as large strings, sound files, video, XML documents . . . the list goes on. . Reference pointers In the C programming language, a reference pointer is a variable containing an address on disk or in memory of whatever the programmer wants to point at. A pointer provides the advantage of not having to specify too much in advance with respect to how many bytes the pointer value occupies. Some relational databases allow the use of pointers where a pointer points to an object or file stored externally to the database, pointing from a field within a table, to the object stored outside the database. Only the address of the externally stored object is stored in the table field. This minimizes structural storage effects on relational tables as often is the result of storing binary objects in table records. Pointers are generally used for pointing to static binary objects. A static object does not change very often. . Collection arrays Some relational databases allow creation of what an object database would call a collection. A collection is a set of values repeated structurally (values are not necessarily the same) where the array is contained within another object, and can only be referenced from that object. In the case of a relational database, the containment factor is the collection being a field in the table. Collection arrays can have storage structures defined in alternative locations to table fields as for binary objects, but do not have to be as such. Collection arrays, much like program arrays, can be either fixed length or dynamic. A dynamic array is a variable-length array, and is actually a pointer. When using a fixed-length array, the programmer must specify the length of the array before using it. . User-defined types Some relational databases allow programmable or even on-the-fly creation of user-defined types. A user-defined type allows the creation of new types. Creation of a new type means that user-defined datatypes can be created by programmers, even using other user-defined types. It follows that fields can be created in tables where those fields have user-defined datatypes. Specialized Datatypes Specialized datatypes take into account datatypes that are intended for contained complex data objects. These specialized datatypes allow types with contained inherent structure (such as XML documents, spatial data, and multimedia objects). Constraints and Validation Relational databases allow constraints, which restrict values that are allowed to be stored in table fields. Some relational databases allow the minimum of constraints necessary to define a database as being a relational database. Some relational databases allow other constraints in addition to the basics. In general, constraints are used to restrict values in tables, make validation checks on one or more fields in a table, or even check values between fields in different tables. Following are some examples of constraints: 47 Database Modeling Building Blocks
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