Re: Duplicate Unique Key constraint error

On Jul 10, 2007, at 3:09 PM, Tom Lane wrote:

>
> "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.

I wonder how SQL server is handling this?  Are they locking the table?
I realize it's off-topic, but I'm still curious.

Sequences are your friend.  they come in INT and BIGINT flavors, but
BIGINT is a lot of rows.

Can set set Sequences to automatically rollover back to zero?