The Network Database Model
The network database model is the first evolved in parallel to the hierarchical database model. One early, a very popular network database was the Integrated Database Management System (IDMS).
Here is a quick introduction to the network database model - what makes it different from or similar to other types of databases:
- Fields are the smallest units of the database structure. Each field also has a unique name.
- Records are the units of database processing (they are memorized, deleted, read); records are the aggregates which are not in a structure of other aggregates.
- Links are hierarchical relations between records of two types. A record of the first type is the owner of the relation, and a record of the second type is an owned member of the other one.
For example, consider records for a PERSON and a PHONENUMBER in a phone directory. PHONENUMBER is owned by the the record of the PERSON.
Data aggregation is a nominate collection of fields or of other data aggregates. For example, ADDRESS would include the following fields: INDEX, HOUSE NUMBER.
The Network Data Model provides a relation between records using direct pointers. Using direct pointers is the fastest method of traversing a network database - what was once known as “walking the set”.
Linked records are organized into the lists. Every owner-record has a pointer to the first owned record in the list. Every owned record has a NEXT pointer: it points at another owned record from the list ( see pict.1).
Different implementations can represent the lists by different ways: from a single-linked list to a double-linked list with a pointer to owner in every owned record (see pict.2). Since pointers in lists are stored directly in the record it makes changing the structure much harder but also extremely fast.
A Network DBMS automatically creates pointers to organize lists of linked records. It is easy to see that double-linked lists would use:
3 * 4 = 12 byte for every owned record and 2 * 4 = 8 bytes for parent records.
If N relations exist for a record then we get an overhead N * 12 bytes.
In practice, many network DBMS did not allow changes to structures. An interesting way to help understand navigation in a network DBMS is the World Wide Web. The World Wide Web consists of links between web pages to other related web pages - circular references are a common occurance.