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

Practical application of unique constraints and unique indexes in PostgreSQL

PostgreSQL's documentation states that unique constraints and unique indexes are functionally equivalent, but recommends using the ALTER TABLE ... ADD CONSTRAINT syntax to enforce unique constraints. This raises questions about potential differences in performance or functionality.

Comparison of unique constraints and unique indexes

To illustrate the difference, let's create a table called master with two columns: con_id with a unique constraint, and ind_id indexed by a unique index.

create table master (
    con_id integer unique,
    ind_id integer
);
create unique index master_idx on master (ind_id);

Uniqueness and foreign keys

Both unique constraints and unique indexes enforce uniqueness on their respective columns. Inserting duplicate values ​​will fail. Foreign keys referencing these columns also work for both types.

Use index as constraint

You can create a table constraint using an existing unique index using the following syntax:

alter table master add constraint master_idx_key unique using index master_idx;

Afterwards, there will be no difference in the description of the column constraints.

Local Index

Unique index declarations allow the creation of partial indexes where a WHERE clause is used to specify a subset of rows to be indexed. However, the unique constraint does not allow this.

Performance and availability

While there is no significant performance difference between unique constraints and unique indexes, the following points may influence your choice:

• Availability: Unique constraints are easier to define and manage than unique indexes.
• Flexibility: Unique indexes allow local indexing and additional index parameters.
• Historical Recommendations: PostgreSQL 9.4 and earlier recommended using unique indexes for unique constraints. However, this suggestion was removed in later versions.

Based on these considerations, the preferred approach in most cases is usually to use unique constraints. For partial indexes or situations where custom index parameters are required, a unique index may be more beneficial. However, the final choice depends on your specific database design requirements and preferences.

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