Practical Guidelines for Supertype and Subtype Modeling - High-Performance Web Databases: Design, Development, and Deployment - page 326

Exhibit 20-6. Metadata table approach.
after implementation. In these cases, the entity constraint matrix provides
the alternate data architecture that the modeler can then implement with
a table- driven application development strategy for the entire system. For
the project data model this means that no subtype entities are included in
any data diagrams. Instead, the entity-relationship diagram is modified to
include a type entity for the supertype (see Exhibit 6). Attributes of the
type entity represent the values in the cells of the entity constraint matrix.
The application system dynamically interrogates the metadata table and
handles instances of the supertype entity according to the constraints
found. Performance considerations are potential concerns because of the
implied additional input-output that the applications require. The model-
ers must develop maintenance capabilities for the physical metadata
tables and procedures to ensure that their contents are valid. In many
cases the benefits of additional flexibility that exist for the organization far
outweigh these concerns. An automated link between the entity constraint
matrix and the table that implements it is highly desirable.
Such approaches to subtype modeling as goal orientation, entity
constraint matrix, and metadata tables offer incremental payback to any