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CODASYL Bachman Data Models DML  

Oracle Tips by Burleson Consulting

High Performance Data Warehousing

The CODASYL Generation Of Database Management 

While the CODASYL Network Model was very good at representing complex data relationships, it had one major drawback: Internal data structures were not transparent to programmers, so programmers were required to navigate database structures to extract information. Unlike a declarative language like SQL, a network database programmer would be required to specify the access path, describing all of the records and the sets that would be used to satisfy the request.

A diagram tool to represent the data structures required by the CODASYL model was popularized by Charles Bachman, and his graphical depiction of the database schema became known as the Bachman diagram, or data structure diagram. In the Bachman diagram, boxes represent records, and arrows represent relationships.

Figure 1.7 The Bachman diagram for describing data models.

The CODASYL model combines two data storage methods to create an engine which can process hundreds of transactions per second. The CODASYL model uses the basic direct access method (BDAM), which utilizes a hashing algorithm (sometimes called a CALC algorithm) to quickly store and retrieve records. CODASYL also employs linked-list data structures, which create embedded pointers in the prefix of each occurrence of a record. These pointers are used to establish relationships among data items. These pointers are called NEXT, PRIOR, and OWNER and are referenced in the Data Manipulation Language (DML). For example, the DML command OBTAIN NEXT ORDER WITHIN CUSTOMER-ORDER would direct the CODASYL database to look in the prefix of the current ORDER record and find the NEXT pointer for the CUSTOMER-ORDER set. The database would then access the record whose address was found at this location.

This is an excerpt from "High Performance Data Warehousing", copyright 1997.

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