Question 1

What causes the 'duplicate key value violates unique constraint users_email_key' error in PostgreSQL?

Accepted Answer

This error is thrown by PostgreSQL's storage engine when an INSERT or UPDATE statement attempts to write a value into the `email` column that already exists in another row. The `users_email_key` constraint is a UNIQUE index enforced at the database level, meaning no application-layer check can prevent it if two concurrent writes race past each other. The fix is to use INSERT ... ON CONFLICT at the SQL level or an ORM upsert method.

Question 2

What is the difference between ON CONFLICT DO NOTHING and ON CONFLICT DO UPDATE in PostgreSQL?

Accepted Answer

ON CONFLICT DO NOTHING silently discards the conflicting INSERT and returns without error or modification — useful when you want idempotent inserts and don't need to update existing data. ON CONFLICT DO UPDATE (upsert) overwrites specified columns of the existing row with the new values using the EXCLUDED pseudo-table. Use DO UPDATE when you need the latest data to win; use DO NOTHING when the existing record should be preserved as-is.

Question 3

Is an application-level SELECT check before INSERT safe for preventing duplicate key errors?

Accepted Answer

No. This is a classic TOCTOU (Time-of-Check to Time-of-Use) race condition. Under concurrent load, two requests can both execute the SELECT, both find no existing row, and both proceed to INSERT — one will succeed and one will hit the constraint violation. The only safe pattern is to let the database enforce uniqueness via the UNIQUE constraint and handle the conflict with ON CONFLICT syntax or catch the 23505 SQLSTATE in your exception handler.

How to Fix PostgreSQL Duplicate Key Violation on Unique Constraint (users_email_key)

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