Re: Solution to UPDATE...INSERT problem - Mailing list pgsql-hackers

"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
> INSERT INTO table SELECT 1, 'foo' EXCEPT SELECT 1, 'foo' FROM table WHERE
> pkcol=1;

> See? So now that INSERT statement will insert the row if it doesn't exist,
> or insert zero rows if it does.  You are then guaranteed that your
> transaction will not fail and rollback, so you can repeat your update, or do
> the insert first and then the update, etc.

Uh, why exactly do you think this is race-free?

It looks fancy, but AFAICS the SELECT will return info that is correct
as of its starting timestamp; which is not enough to guarantee that the
INSERT won't conflict with another transaction doing the same thing
concurrently.

            regards, tom lane