Composite vs. Surrogate Keys for Many-to-Many Tables: Which Primary Key is Best for Performance?

SQL: Primary Keys for Many-to-Many Tables

In database design, the choice between using a composite primary key or an auto-increment surrogate key for a many-to-many table is a topic of debate. Some argue for the efficiency of a surrogate key, while others prefer the simplicity of a composite key.

Composite Primary Key

A composite primary key consists of the foreign keys from both tables involved in the many-to-many relationship. This guarantees uniqueness and eliminates the need for additional indexes. However, when inserting a new record, the database may have to reorganize the table data to maintain the sorted order based on the primary key.

Auto-Increment Surrogate Key

An auto-increment surrogate key is a unique identifier that is automatically generated for each new record. This simplifies indexing and allows new records to be added to the end of the table, reducing the need for data reorganization. However, it introduces an additional column and adds a potential source of data redundancy.

Performance Considerations

According to a commentator in the question, performance concerns arise from table reorganization when using a composite primary key. However, this argument is flawed. Modern databases use efficient data structures like B-trees and balanced multi-way trees to store and retrieve data quickly, making data reorganization a negligible factor.

Read-Write Frequency

Database tables are typically read more frequently than they are written. This makes selecting performance optimizations more critical. A composite primary key provides efficient access for both reads and writes, while a surrogate key only benefits writes by avoiding table reorganization.

Conclusion

Both composite primary keys and auto-increment surrogate keys can be used for many-to-many tables. However, the simplicity and performance benefits of a composite primary key make it the preferred choice in most cases. When read performance is paramount, or index maintenance is a concern, a composite primary key is the recommended option.

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