Re: Duplicate Unique Key constraint error

"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.

            regards, tom lane