On Fri, Dec 30, 2011 at 4:19 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Jan Bakuwel <jan.bakuwel@greenpeace.org> writes:
>> Why-o-why have the PostgreSQL developers decided to do it this way...?
>
> Because starting and cleaning up a subtransaction is an expensive thing.
> If we had auto-rollback at the statement level, you would be paying that
> overhead for every statement in every transaction, whether you need it
> or not (since obviously there's no way to forecast in advance whether a
> statement will fail). Making it depend on explicit savepoints allows
> the user/application to control whether that overhead is expended or
> not.
>
> If you want to pay that price all the time, there are client-side
> frameworks that will do it for you, or you can roll your own easily
> enough. So we do not see it as a big deal that the database server
> itself doesn't act that way.
Having used PostgreSQL a LOT, I find that being able to throw an
entire update at the db and having it fail / be rolled back / CTRL-C
out of and fix the problem is actually much less work than the
frameworks for other databases. Once you've chased down bad data in a
load file a few times, it's really pretty easy to spot and fix these
issues and just run the whole transaction again. Since PostgreSQL
doesn't have a very big penalty for rolling back a whole transaction
it's not that bad. Some dbs, like MySQL with innodb table handler
have a 10:1 or greater penalty for rollbacks. Insert a million rows
in innodb then issue a rollback and go get a sandwich. In PostgreSQL
a rollback is generally instantaneous, with the only real cost being
bloat in the tables or indexes.