Fixing PostgreSQL Error: Duplicate Key Violations

The Problem

Duplicate key violations occur in PostgreSQL when an INSERT or UPDATE operation attempts to insert or modify a record that would violate a unique constraint, commonly due to a primary key or unique index. Catching and handling these errors is crucial to maintaining data integrity and application stability.

Solution 1: Use ON CONFLICT Clause

With PostgreSQL, the ON CONFLICT clause allows you to define an alternative action when a duplicate key violation is encountered, such as skipping the insertion or updating the existing row.

1. Identify the column or columns with the unique constraint.
2. Write an INSERT statement including an ON CONFLICT clause specifying the conflict target and the alternative action.
3. Execute the INSERT statement in your PostgreSQL client or application code.

ExanokeL

INSERT INTO your_table (id, column1, column2)
VALUES (1, 'data1', 'data2')
ON CONFLICT (id) DO NOTHING;

Performance discussion: Using ON CONFLICT DO NOTHING avoids unnecessary exceptions which can be costly in terms of performance. However, ensuring that this clause is used sparingly and appropriately is essential.

Advantages: It prevents exceptions, maintains application flow, and is efficient for handling insertions when duplicate keys are not significant.
Limitations: It does not inform you of ignored conflicts, and thus, if the duplicate key was unintentional, it could lead to data inconsistency without proper monitoring.

Solution 2: Perform a Manual Check Before Insertion

Before attempting an INSERT operation, make a SELECT query to check for existing records that might cause a duplicate key error.

1. Write a SELECT statement to check if the record already exists based on the unique key.
2. Conditional logic in the application layer to decide if INSERT should proceed.
3. Continue with the INSERT command if no record is found.

Example:

SELECT EXISTS (
    SELECT 1 FROM your_table WHERE id = 1
);

-- If the above command returns false, proceed with the insert
INSERT INTO your_table (id, column1, column2)
VALUES (1, 'data1', 'data2');

Performance discussion: This solution may lead to a race condition where the record is inserted by another transaction after the check but before the INSERT. This can be prevented by using appropriate transaction isolation levels.

Advantages: Application gains full control over the decision to insert.
Limitations: Race conditions can occur; it’s less efficient as it adds an extra query prior to every INSERT.

Solution 3: Update Or Insert (Upsert)

When a duplicate entry is detected, rather than discarding it, the data can be used to update the existing record. This strategy is often referred to as an ‘upsert’ (update + insert).

1. Identify fields for detecting a duplicate entry.
2. Use the ON CONFLICT clause to define an UPDATE action.
3. Execute the upsert command.

Example:

INSERT INTO your_table (id, column1, column2)
VALUES (1, 'newdata1', 'newdata2')
ON CONFLICT (id) DO UPDATE SET
  column1 = EXCLUDED.column1,
  column2 = EXCLUDED.column2;

Performance discussion: This combines two operations into one, which can be more performant than separate UPDATE and INSERT commands while also taking care of concurrency issues.

Advantages: Reduces the number of queries and handles conflicts elegantly by ensuring data is always up-to-date.
Limitations: Not suitable if the record should not be altered after initial insertion or in cases where the business logic demands a notification of an attempted duplicate insertion.