Thanks for the clarification guys! That was not the behavior I was expecting (as you can tell), so I learned something new today. :)
In my case I don't want an update (there are only the 2 fields, so it's just insert or delete), so I'll fire the insert as it is (that'll get the cases where it's not a concurrent update failure) and catch the failure to verify that the data exists - if it does, I'll ignore the failure; if not, i'll throw an exception. Larry On Sat, Jun 27, 2015 at 10:57 PM Pavel Stehule <pavel.steh...@gmail.com> wrote: > 2015-06-28 6:52 GMT+02:00 Peter Geoghegan <peter.geoghega...@gmail.com>: > >> On Sat, Jun 27, 2015 at 9:47 PM, Pavel Stehule <pavel.steh...@gmail.com> >> wrote: >> > you can protect it against this issue with locking - in this case you >> can >> > try "for update" clause >> > >> > http://www.postgresql.org/docs/9.4/static/explicit-locking.html >> > >> > insert into Favorite (patronId, titleId) >> > select 123, 234 >> > where not exists ( >> > select 1 from Favorite where patronId = 123 and titleId = 234 for >> update >> > ) >> >> That won't work reliably either -- a SELECT ... FOR UPDATE will still >> use an MVCC snapshot. The looping + subxact pattern must be used [1] >> if a duplicate violation isn't acceptable. ON CONFLICT DO UPDATE >> should be preferred once 9.5 is released. >> >> [1] >> http://www.postgresql.org/docs/9.4/static/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE >> > > yes, you have true - cannot to lock, what doesn't exists in pg > > Regards > > Pavel > > > > >> -- >> Regards, >> Peter Geoghegan >> > >
