Unique Constraint vs. Unique Index in PostgreSQL: Which Should You Choose?

Unique constraints and unique indexes in PostgreSQL: a choice between style and performance

In PostgreSQL, unique constraints or unique indexes can be used to define uniqueness. However, the documentation suggests that the preferred method is to use the ALTER TABLE ... ADD CONSTRAINT syntax.

Define uniqueness

To illustrate the equivalence between the two, consider the following code:

<code class="language-sql">-- 使用唯一约束
create table foo (
    id serial primary key,
    code integer,
    label text,
    constraint foo_uq unique (code, label)
);

-- 使用唯一索引
create table foo (
    id serial primary key,
    code integer,
    label text
);
create unique index foo_idx on foo using btree (code, label);</code>

As stated in the PostgreSQL documentation, these two methods are functionally equivalent. Both methods will prevent duplicate values ​​for the specified column.

Style and implementation details

The preferred way to define unique constraints is through the ALTER TABLE ... ADD CONSTRAINT syntax. This is considered best practice and follows PostgreSQL's recommended approach. While using a unique index to enforce uniqueness still works, this is considered an implementation detail that should not be accessed directly.

Practical impact

In terms of performance and functionality, there is no significant practical difference between the two methods. Both options provide the same level of data integrity and uniqueness enforcement.

However, there are some differences to consider:

• Foreign key: Unique constraints can be directly referenced by foreign keys, but unique indexes cannot.
• Constraints using indexes: Unique constraints can be created using existing unique indexes, effectively eliminating functional differences.
• Partial Index: A unique index can be a partial index, allowing the index to be created on a subset of the data. However, table constraints cannot use partial indexes.

Conclusion

While the choice between defining uniqueness using a unique constraint or a unique index ultimately comes down to style and preference, it is recommended to follow PostgreSQL's preferred approach, which is to use the ALTER TABLE ... ADD CONSTRAINT syntax.

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