Composite or Surrogate Primary Key for Many-to-Many Tables: Which Performs Better?

Many-to-Many Table Primary Key: Composite vs. Surrogate

In the realm of database design, many-to-many relationships pose a question: should the primary key of the mapping table be a composite of the foreign keys from the related tables, or an auto-incrementing surrogate key?

Disadvantages of Surrogate Keys

According to the argument against surrogate keys, creating a composite primary key (PartID, DeviceID) mandates physical disk sorting in that order. Inserting a new record (Part1/Device3) between existing entries (Part1/Device1) and (Part2/Device3) would require significant data shuffling, becoming problematic for large table sizes.

Benefits of Composite Primary Keys

Proponents of composite primary keys dismiss these concerns, claiming that:

• Composite keys guarantee uniqueness, negating the need for an additional surrogate key.
• Indexes created on (col2, col1) can compensate for cases where reversing the order of the primary key columns would improve performance.
• Individual column indexes are unnecessary since the table serves solely for joining the two referenced tables.

Performance Implications

The performance implications of choosing between composite and surrogate keys are minimal for two-column many-to-many mappings. However, for more complex mappings or heavy insert workloads, surrogate keys may offer advantages, such as:

• Performance for Inserts: Surrogate keys allow for faster inserts by avoiding the need to split and rearrange existing data.
• Reduced Index Fragmentation: By avoiding primary key updates, surrogate keys reduce index fragmentation, improving query performance.
• Compactness: Surrogate keys are typically smaller than composite keys, optimizing space utilization.

Conclusion

In most cases, the choice between composite and surrogate keys for many-to-many tables is a matter of preference. For simple two-column mappings, composite primary keys offer simplicity and uniqueness. However, for complex mappings or high insert workloads, surrogate keys may provide better performance and data integrity.

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