"Harpreet Dhaliwal" < harpreet.dhaliwal01@gmail.com> writes: > Transaction 1 started, saw max(dig_id) = 30 and inserted new dig_id=31. > Now the time when Transaction 2 started and read max(dig_id) it was still 30 > and by the time it tried to insert 31, 31 was already inserted by > Transaction 1 and hence the unique key constraint error.
This is exactly why you're recommended to use sequences (ie serial columns) for generating IDs. Taking max()+1 does not work, unless you're willing to lock the whole table and throw away vast amounts of concurrency.