However, SELECT FOR UPDATE doesn't lock not-yet-existing rows in PostgreSQL, like it does in MySQL with gap locks. If for example, the transaction performs either an UPDATE or an INSERT based on whether the SELECT FOR UPDATE found or didn’t find a row, then two concurrent executions of that transaction can run into a conflict or duplicate INSERT in PostgreSQL, whereas in MySQL the second execution would block on the SELECT FOR UPDATE of the first execution even when there are no matching rows (but the first execution then will/might create some).