How to Guarantee Consistent Results from PostgreSQL UPSERT Operations with RETURNING?

PostgreSQL UPSERT Operations and the RETURNING Clause: Handling Conflicts

PostgreSQL's INSERT ... ON CONFLICT provides upsert functionality, combining insertion and updates. However, using DO NOTHING with the RETURNING clause can lead to incomplete results in concurrent scenarios.

Concurrency Problem 1: Missing Returned Rows

If another transaction modifies the target row before your INSERT ... ON CONFLICT completes, the upsert might not detect the conflict, resulting in missing rows in the RETURNING output.

Solutions:

Several approaches mitigate this:

• Row Count Verification: Compare the input row count with the RETURNING count. Discrepancies indicate missing rows, prompting a re-execution of the statement.
• Forceful Overwrite: A separate CTE (Common Table Expression) can insert any missing rows. While effective, this method risks deadlocks with multiple overlapping transactions.

Concurrency Problem 2: Row Locking

For transactions requiring row locks, use ON CONFLICT DO UPDATE with WHERE FALSE. This locks rows without altering them. Combine this with SELECT ... FOR UPDATE for additional locking.

Data Type Handling and Casting for Robustness

Existing solutions are insufficient for all concurrent scenarios. A more comprehensive approach involves:

Low Concurrency:

• Employ a CTE with INSERT and SELECT UNION to differentiate between inserted and selected rows.
• Use a CTE to define input data types, aligning them with the target table schema.

High Concurrency:

• Use the low-concurrency approach.
• Include a CTE to forcefully insert missing rows if needed.
• Lock relevant rows within the INSERT ... ON CONFLICT CTE to manage concurrency.

Deadlock Avoidance:

Consistent insertion order minimizes deadlock occurrences.

Data Type Management:

• Leverage an existing relation to automatically determine input data types.
• Omit column names for automatic column mapping.

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