On Sat, 11 Sep 2004 11:27:02 -0400, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> > There is no "between" a single statement.
>
> Sure there is. In the above example, the EXISTS result will be correct
> as of the time of the snapshot that was taken at the start of the
> command (or the start of the whole transaction, if using SERIALIZABLE
> mode). So it is *entirely* possible for the INSERT to fail on duplicate
> key if some other transaction commits a conflicting row concurrently.
>
> AFAIK, all the bulletproof solutions for this sort of problem involve
> being prepared to recover from a failed insertion. There are various
> ways you can do that but they all come down to needing to catch the
> duplicate key error. In the past you have had to code that in
> client-side logic. In 8.0 you could write a plpgsql function that
> catches the exception.
>
> Given the need for a test anyway, I think the WHERE NOT EXISTS above
> is pretty much a waste of time. Just do an INSERT, and if it fails do
> an UPDATE; or do an UPDATE, and if it fails (hits zero rows) then do
> an INSERT, being prepared to go back to the UPDATE if the INSERT fails.
> Which of these is better probably depends on how often you expect each
> path to be taken.
It's not meant to be a bulletproof solution. It's meant to be a
syntactically equivalent to the MySQL statement. You still have to
check for a failure.
Do the update followed by the insert in a serial transaction. If the
transaction fails you redo the same SQL transaction. This eliminates
the need for a this query else this query scenario which is the whole
point of the MySQL bastard syntax in the first place. Not the best
solution but if you have a good DBA and bad programmers it might be
what you actually want.