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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