Composite Key vs. Surrogate Key in Many-to-Many Tables: Which Primary Key Design is Best?

Primary Key Design in Many-to-Many Table

The use of composite primary key versus surrogate auto-increment primary key in many-to-many tables has been a subject of debate. While composite primary keys guarantee uniqueness by combining foreign keys, surrogate keys introduce an additional, auto-generated ID field.

Some argue that surrogate keys provide better performance for inserts, as new records can be appended to the end without disrupting the table order. However, this argument is based on an outdated understanding of database storage and indexing.

In reality, database tables are stored and indexed using balanced multi-way tree structures, not arrays. This means that inserts can be performed efficiently regardless of the primary key design. Additionally, indexes are used to optimize queries, making the order of table rows irrelevant.

As a result, the benefits of using a composite primary key outweigh those of a surrogate key. Composite primary keys:

• Guarantee uniqueness without requiring an additional field
• Enable efficient joining of referenced tables using the primary key index
• Eliminate the need for indexes on individual foreign key columns

Therefore, for simple two-column many-to-many mappings, it is recommended to use a composite primary key on the foreign key columns. This provides both uniqueness and optimal performance for both read and write operations.

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