On Sat, Jun 27, 2015 at 9:47 PM, Pavel Stehule <pavel.stehule@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
--
Regards,
Peter Geoghegan