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

Postgres unique constraints and indexes

Introduction

Unique constraints and indexes are both used to ensure the uniqueness of data in database tables. However, there are some key differences between the two approaches.

Unique constraints and indexes in Postgres

Unique Constraint

• Explicitly defined in the table definition using the CONSTRAINT keyword.
• Enforce uniqueness by preventing duplicate values ​​from being inserted into the columns specified in the constraint.
• can be used to create primary keys.
• Always applies to all rows in the table.

Index

• Created using the CREATE INDEX statement.
• Speed ​​up queries by providing a faster way to locate data in a table.
• Can be unique or non-unique.
• Can be applied to all rows or only to a subset of rows (partial index).

Recommended method

According to the Postgres documentation, using ALTER TABLE ... ADD CONSTRAINT is the preferred way to add unique constraints to a table. This is because creating unique indexes specifically to enforce uniqueness is considered an implementation detail and should not be accessed directly.

Practical results

Performance: Generally, unique constraints and unique indexes perform similarly in terms of query speed. However, partial indexes can improve the performance of queries that access only a subset of the data.

Foreign keys: Unique constraints can be referenced by foreign keys, but unique indexes cannot.

Constraint Overhead: Unique constraints have a slight overhead compared to unique indexes because they require additional maintenance operations when inserting or updating data.

Example

To illustrate the difference between a unique constraint and an index, let’s create a table with both a unique constraint and a unique index:

CREATE TABLE foo (
    id SERIAL PRIMARY KEY,
    code INTEGER,
    label TEXT,
    CONSTRAINT foo_uq UNIQUE (code, label)
);
CREATE UNIQUE INDEX foo_idx ON foo (code, label);

Both the unique constraint (foo_uq) and the unique index (foo_idx) will enforce uniqueness of the (code, label) combination. However, you should prefer using unique constraints as it is the recommended approach in Postgres.

Partial Index

To create a partial index, use the CREATE INDEX clause within the WHERE statement:

CREATE UNIQUE INDEX foo_partial_idx ON foo (code) WHERE label IS NOT NULL;

This index only applies to rows where label is not NULL .

Add constraints using indexes

You cannot add unique constraints using partial indexes. However, you can create a unique constraint using an existing unique index:

ALTER TABLE foo ADD CONSTRAINT foo_partial_uq UNIQUE USING INDEX foo_partial_idx;

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