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

Detailed explanation of PostgreSQL unique constraints and unique indexes

PostgreSQL provides two methods to enforce data uniqueness on a table: unique constraints and unique indexes. Although the documentation states that they are equivalent, a note in earlier versions warned against using indexes for unique constraints.

Equivalence and Difference

The code snippet provided shows that unique constraints and unique indexes are essentially equivalent in enforcing uniqueness. Both will prevent duplicate values ​​for the specified column. However, there are subtle differences:

• Table constraints and index constraints: Unique constraints are declared as part of the table definition, while unique indexes are defined separately. This affects foreign key references, because foreign keys can reference either of these two constraint types.
• Supported features: Unique indexes are more flexible than table constraints. They allow partial indexing, expression indexing, and other advanced options.

Practical considerations

In terms of performance, there is usually no significant difference between using unique constraints and unique indexes. However, unique constraints may have slightly better performance in foreign key operations because the foreign key references the actual constraint rather than the index.

Style and Best Practices

While it is technically valid to use a unique index to enforce a unique constraint, it is recommended to use table constraints. Table constraints are more explicit and self-documenting, and they are consistent with the preferred workflow for adding unique constraints using ALTER TABLE ... ADD CONSTRAINT.

In summary, both unique constraints and unique indexes can ensure data uniqueness in PostgreSQL. Although unique indexes provide greater flexibility, in some cases table constraints are preferred for their simplicity and better performance.

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