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 >
