CODEWITHSUNDEEP
databasemodelrelational-databasehierarchical
SANM2009
SANM2009

Reputation: 1998

hierarchical database model - physical pointers

I am reading a book about databases and I was wondering if someone could explain something that I can't understand / visualise. I have used relational databases and understand foreign keys etc. My question is, what is a "physical pointer" in a hierarchical data model?

The earlier hierarchical data model is suitable for data structures that are naturally hierarchical, with each data structure placed at a certain level in the hierarchy. However, in the business arena, many of the data structures and their relationships cannot be readily placed in a hierarchical arrangement. The network data model evolved to dispense with the arbitrary restriction of the hierarchical model. Nevertheless, in both of these models, you need physical pointers to connect related data occurrences. This is a serious drawback because you have rewrite the physical addresses in the data records every time you reorganize the data, move the data to a different storage area, or change over to another storage medium. The relational model establishes the connections between related data occurrences by means of logical links implemented through foreign keys.

Ponniah, P. (2003) Database Design and Development: An Essential Guide for IT Professionals [Online], Hoboken, New Jersey, John Wiley & Sons. Available at http://onlinelibrary.wiley.com/book/10.1002/0471728993

Upvotes: 0

Views: 1030

Answers (1)

reaanb
reaanb

Reputation: 10066

A physical pointer is any value that indicates where something is located, e.g. a memory address, position in an array or collection, a file system path, OID, etc.

In contrast, the relational model identifies things by their own attributes - natural keys or assigned surrogate keys. This adds a layer of abstraction and complexity to the DBMS, but isolates users from the organization of system data structures.

For example, in hierarchical data structures, each child record might contain the address of its parent record, while a parent record might hold an array of its children's addresses.

Upvotes: 2

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