Unique Constraints vs. Unique Indexes in PostgreSQL: When Should You Use Which?

Unique constraints vs. unique indexes in PostgreSQL: When to use which?

In PostgreSQL, unique constraints and unique indexes are generally considered interchangeable. However, the manual recommends that it is best to use the ALTER TABLE ... ADD CONSTRAINT syntax to define unique constraints.

Understand the differences

Although they appear to be equivalent, there are subtle differences between unique constraints and unique indexes:

• Creation source: Unique constraints are created using the CONSTRAINT keyword, while unique indexes are created using the INDEX keyword.
• Foreign key compatibility: Unique constraints can be directly referenced by foreign keys, while unique indexes cannot.
• Partial indexes: Unique indexes allow the creation of partial indexes based on the WHERE clause, while constraints do not support partial indexes.
• Expression index: Unique indexes can be defined on expressions, while constraints cannot.

Practical considerations

Using unique constraints or unique indexes depends on the specific requirements of the table and application.

• Performance: In most cases, unique constraints and unique indexes perform similarly in terms of data insertion and retrieval speed. However, partial indexes created using only unique indexes can improve the performance of queries that filter on specific values.
• Readability and Maintainability: Unique constraints are explicitly declared as part of the table definition, making it easier to read and understand. Unique indexes, on the other hand, are separate entities and may require additional work to identify their purpose.
• Upgrade: In PostgreSQL 9.4, a note in the manual discourages the use of unique indexes to enforce uniqueness. However, this note was removed in version 9.5, indicating that this was not a strict requirement, but a preference for consistency and ease of management.

Conclusion

Although unique constraints and unique indexes share some functionality, they have different characteristics and uses. Understanding these differences can help you make informed decisions when creating tables in PostgreSQL, balancing various factors such as performance, readability, and future maintenance.

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