Why Does Postgres Throw a 'No Unique Constraint Matching Given Keys' Error When Creating Foreign Keys?

Understanding Postgres Referential Integrity Errors

When building database structures in Postgres (version 9.1 or later), you might encounter this error:

<code>ERROR: there is no unique constraint matching given keys for referenced table "bar"</code>

This error arises from Postgres's enforcement of referential integrity, a crucial aspect of database consistency. Let's break down the cause and solution.

Referential Integrity and Foreign Keys

Referential integrity ensures data consistency across related tables. Foreign key relationships establish links between tables. For example, if table bar has a foreign key foo_fk referencing the name column in table foo, referential integrity mandates that every foo_fk value in bar must correspond to an existing name value in foo.

The Role of Unique Constraints

To uphold referential integrity, Postgres requires a unique constraint on the referenced column (name in table foo). This constraint guarantees that each name value in foo is unique, allowing Postgres to unambiguously identify the referenced row.

Error Cause: Missing Unique Constraint

The error message indicates a missing unique constraint on the column referenced by the foreign key. If the name column in foo lacks a unique constraint, Postgres cannot reliably link foo_fk values in bar to specific rows in foo, resulting in the error.

Resolving the Error: Adding a Unique Constraint

The solution is straightforward: add a unique constraint to the referenced column. In SQL, this is done using the ALTER TABLE and ADD CONSTRAINT commands:

ALTER TABLE foo ADD CONSTRAINT foo_name_unique UNIQUE (name);

This addition enforces uniqueness for the name column in foo, satisfying Postgres's requirements for referential integrity and enabling the foreign key relationship to function correctly. After adding the constraint, attempts to create the foreign key relationship should succeed.

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