How Does PostgreSQL's Deferred vs. Immediate Primary Key Constraint Enforcement Affect Transaction Behavior?

Enforcement of DEFERRABLE vs. IMMEDIATE Primary Key Constraints

The enforcement of a deferred/deferrable unique or primary key constraint in PostgreSQL depends on its settings and the type of operation being performed.

For constraints defined as DEFERRABLE INITIALLY IMMEDIATE, uniqueness is checked after the execution of each SQL statement. However, it is important to note that for UNIQUE or PRIMARY KEY constraints, the manual states that uniqueness checks occur immediately after every command, regardless of the deferrability setting.

The example provided in the query demonstrates that an UPDATE statement modifying multiple rows is allowed to succeed even though the constraint is defined as PRIMARY KEY DEFERRABLE INITIALLY IMMEDIATE. This is because the check occurs after the statement execution, and in this case, the constraint is still satisfied.

In contrast, a data modifying CTE that attempts to update rows in multiple tables can fail if the PK constraint is not deferred. This is because each sub-statement in the CTE is executed concurrently, and without snapshot isolation, the order of updates is not predictable. As a result, a unique key violation can occur.

If multiple UPDATE statements are executed within a single transaction without explicitly setting constraints as deferred, a UNIQUE violation can occur if the constraint is not deferred. This is because the checks are performed after each statement, and the constraint may be violated during intermediate states of the transaction.

Therefore, it is evident that the behavior of non-deferrable UNIQUE or PRIMARY KEY constraints in PostgreSQL is essentially flawed. The checks are performed after each row updated, which can lead to unexpected failures in certain scenarios. However, the workaround for this issue is to use a DEFERRABLE constraint, which provides more flexibility in enforcing uniqueness and can prevent unnecessary errors.

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