Обсуждение: do I need a rollback() after commit that fails?

I'm running Pg 8.3.7 on FreeBSD 7.2.

I have some code in Perl that does a bunch of inserts and updates with
all constraints deferred.  On occasion, one of the FK's gets violated
and the transaction commit fails.

I trap this with code like this:

    unless ($dbh->commit()) {
      warn "commit failure ".$dbh->errstr;
      $dbh->rollback();
      return 'failed';
    }

The DBI is telling me that the rollback() is useless with AutoCommit
is on (which it is).

I did some direct testing with psql and it seems that this is not Perl
DBI specific behavior.

So, it seems that if commit fails, I don't need to issue a rollback.
Is this portable to other databases, or is this Postgres specific?

I also note that if I do not defer the constraints, and issue the
commit even after the INSERT reports error, that the statement result
printed by commit is instead "ROLLBACK".  If I have constraints
deferred, the commit output is just the "ERROR" statement, without any
indication of ROLLBACK.

On Tue, Sep 29, 2009 at 1:26 PM, Vick Khera <vivek@khera.org> wrote:
> I'm running Pg 8.3.7 on FreeBSD 7.2.
>
> I have some code in Perl that does a bunch of inserts and updates with
> all constraints deferred.  On occasion, one of the FK's gets violated
> and the transaction commit fails.
>
> I trap this with code like this:
>
>    unless ($dbh->commit()) {
>      warn "commit failure ".$dbh->errstr;
>      $dbh->rollback();
>      return 'failed';
>    }
>
> The DBI is telling me that the rollback() is useless with AutoCommit
> is on (which it is).
>
> I did some direct testing with psql and it seems that this is not Perl
> DBI specific behavior.
>
> So, it seems that if commit fails, I don't need to issue a rollback.
> Is this portable to other databases, or is this Postgres specific?
>
> I also note that if I do not defer the constraints, and issue the
> commit even after the INSERT reports error, that the statement result
> printed by commit is instead "ROLLBACK".  If I have constraints
> deferred, the commit output is just the "ERROR" statement, without any
> indication of ROLLBACK.
>

If a query within a transaction has errors, COMMIT at the end of the
transaction is automatically turned into a ROLLBACK.

Vick Khera wrote:
> I'm running Pg 8.3.7 on FreeBSD 7.2.
>
> I have some code in Perl that does a bunch of inserts and updates with
> all constraints deferred.  On occasion, one of the FK's gets violated
> and the transaction commit fails.
>
> I trap this with code like this:
>
>     unless ($dbh->commit()) {
>       warn "commit failure ".$dbh->errstr;
>       $dbh->rollback();
>       return 'failed';
>     }
>
> The DBI is telling me that the rollback() is useless with AutoCommit
> is on (which it is).
>

Unless I'm mistaken, if AutoCommit is enabled, then each statement will
be commit for you.  The commit() and the rollback() are both useless.

-Andy

On Tue, Sep 29, 2009 at 3:56 PM, Andy Colson <andy@squeakycode.net> wrote:
> Unless I'm mistaken, if AutoCommit is enabled, then each statement will be
> commit for you.  The commit() and the rollback() are both useless.
>

To clarify, the DBI driver turns off AutoCommit in postgres when
begin_work() is called.  It does not however disable its internal
AutoCommit flag.

The question still stands: if the COMMIT fails, ROLLBACK is not
required in Postgres.  Is this portable to other databases?

On 30 Sep 2009, at 4:01, Vick Khera wrote:

> The question still stands: if the COMMIT fails, ROLLBACK is not
> required in Postgres.  Is this portable to other databases?


I don't think so. I recall messages on this list claiming that some
databases (MS SQL, MySQL if memory serves me) commit the queries up to
the failed query anyway if you issue a COMMIT (which is just wrong!),
so the commit succeeds and there's nothing to rollback after that.
Some searching should turn up those messages, if I recall correctly
the issue at hand was that people expected that behaviour in Postgres
too.

But I don't know what Perl DBI does internally when issuing $dbh-
 >commit(), maybe it's taking such things into account already.

Alban Hertroys

--
Screwing up is the best way to attach something to the ceiling.


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