Comment by saurik

...and then you additionally have to verify that the specific reason both the update and the insert failed was due to concurrency, or you can make the mistake I did a couple years ago where I had transactions spinning against my database for weeks on end due to an unrelated constraint failure I had failed to notice with some transactions that I kept retrying in a loop as part of my hacked together upsert implementation (which is why my user id numbers jumped from ~8m to ~460m... it is a meaningless mistake, but one I am reminded of every day due to that "46" staring at me :/).