Primary Key Design in Many-to-Many Tables: Performance Implications

In the realm of database design, constructing many-to-many relationships presents a key question: should the primary key consist of a composite of the foreign keys or an auto-incrementing surrogate?

Composite vs. Surrogate Primary Key

Option 1: Composite Primary Key on Foreign Keys

• PartID: Foreign key from the parent Part table
• DeviceID: Foreign key from the parent Device table

Option 2: Auto-Incrementing Surrogate Primary Key

• ID: Auto-incrementing unique identifier
• PartID: Foreign key
• DeviceID: Foreign key

Performance Considerations

The provided comment emphasizes the potential performance implications of using a composite primary key, claiming it leads to physical table sorting and inefficient insertions.

Analysis of the Comment

However, according to expert opinion, this comment is flawed. Modern databases employ advanced data structures (balanced multi-way trees) rather than simple arrays for storing data. This eliminates the need for sequential storage or reorganizing upon insertions.

Moreover, in real-world scenarios, database tables are predominantly read rather than written. Hence, optimizing the indexing for efficient retrieval is more critical than minimizing insertion times.

Conclusion

For simple two-column many-to-many mappings, a composite primary key with no surrogate column is recommended. It ensures uniqueness without sacrificing space and allows for efficient indexing on both foreign keys for optimal read performance.