Does PostgreSQL Automatically Index Foreign Keys?

Indexes on Foreign Keys and Primary Keys in Postgres

In PostgreSQL, certain database objects automatically receive indexes, while others may require manual creation. This article investigates the specific cases where PostgreSQL automatically generates indexes and provides a method to identify all indexes associated with a table.

Auto-Indexed Objects

PostgreSQL automatically creates indexes on primary keys and unique constraints. By enforcing uniqueness, these indexes play a crucial role in ensuring data integrity.

Identifying Auto-Generated Indexes

PostgreSQL notifies users of any dynamically created indexes with a NOTICE-level message. These notifications appear in both the psql console and system logs. Moreover, the d command displays auto-generated indexes within its output.

Foreign Key Indexes

Despite automatically indexing primary keys and unique constraints, PostgreSQL refrains from indexing foreign key references. This omission stems from the fact that creating an index on referencing columns is not always necessary and can introduce performance concerns.

Creating Foreign Key Indexes

If deemed beneficial, users can manually create indexes on foreign key columns. However, it is important to consider the potential impact on DML operations, as each index incurs a slight performance overhead during INSERTs, UPDATEs, and DELETEs.

Additional Considerations

In the context of primary-foreign key relationships, where two foreign keys constitute a primary key, an index is automatically created on the primary key. In such cases, additional indexes on the foreign key columns may be superfluous.

Determining the need for a referencing-side index on foreign keys requires careful analysis. While generally recommended, it ultimately depends on the frequency of index usage.

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