Composite vs. Surrogate Primary Key in Many-to-Many SQL Tables: Which is Better?

SQL Many-to-Many Table Primary Key

In a many-to-many table, there are two opposing approaches to defining the primary key:

Composite Primary Key

• Consists of the two foreign key columns
• Ensures uniqueness and preserves the record ordering

Surrogate Primary Key

• Auto-incrementing column used as a unique identifier
• Allows records to be appended to the end of the table

Performance Implications

The choice between these two approaches revolves primarily around performance. The commenter suggests that a surrogate key is preferable as it avoids the need for table re-organization during record insertions.

However, it's crucial to note that modern databases utilize highly optimized data structures (e.g., balanced multi-way trees) for index storage and retrieval. Therefore, the performance difference between a composite key (with an index on the reversed order) and a surrogate key is negligible for most practical purposes.

Specifically, the following benefits of a composite primary key outweigh any potential performance drawbacks:

• No additional space overhead: Surrogate keys consume additional storage space, which can become significant for massive datasets.
• Guaranteed uniqueness: The combination of foreign key columns is guaranteed to be unique, reducing the need for additional indexing or constraints.
• Selective indexing: An index on the reversed order can handle queries efficiently, even when the reversed ordering is preferred for certain operations.

Conclusion

While a surrogate primary key may seem appealing from a theoretical standpoint, empirical evidence suggests that a composite primary key is more performant and efficient in most practical SQL scenarios. The absence of space overhead, guaranteed uniqueness, and selective indexing make the composite approach the preferred choice for many-to-many table primary keys.

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