Обсуждение: perl dbd libraries keeping transactions open?

    I'm not sure if this is the right place, but I've got
a question and potential issue.
We've got a system consisting of several daemons written
in perl and a web interface, and we are having issues with
the nightly vacuums not doing anything.  It seems that this
is happening because there is always a transaction held open
by the libraries.
It seems that the implementation is for the libs to
do a BEGIN on connect and after every transaction -- is
this normal.  Is there a way to keep it from causing
problems with vaccuums?
If this isn't the right place, or if anyone knows a
better place to ask this, let me know...

-- 
Adam Haberlach         | "Because manholes are round."
adam@mediariffic.com   |
http://mediariffic.com |

Adam Haberlach <adam@newsnipple.com> writes:
>     It seems that the implementation is for the libs to
> do a BEGIN on connect and after every transaction -- is
> this normal.  Is there a way to keep it from causing
> problems with vaccuums?

This is horrible practice for a number of reasons, not only its effect
on VACUUM.  Consider what you will get from now(), for example.  A new
BEGIN should only be issued when you are ready to issue the first
command of the next transaction.

If that hasn't been fixed yet in the dbd driver, I would recommend
fixing it there.
        regards, tom lane

On Sunday 02 March 2003 18:55, Tom Lane wrote:
> Adam Haberlach <adam@newsnipple.com> writes:
> >     It seems that the implementation is for the libs to
> > do a BEGIN on connect and after every transaction -- is
> > this normal.  Is there a way to keep it from causing
> > problems with vaccuums?
>
> This is horrible practice for a number of reasons, not only its effect
> on VACUUM.  Consider what you will get from now(), for example.  A new
> BEGIN should only be issued when you are ready to issue the first
> command of the next transaction.
>
> If that hasn't been fixed yet in the dbd driver, I would recommend
> fixing it there.

The default DBD::Pg behaviour is AutoCommit On, and AFAIK
has always been that way. The DBD::Pg driver will only
begin a transaction if AutoCommit was explicitly set to Off
or $dbh->begin_work() was executed by the application.

perldoc DBD::Pg contains more info.

Ian Barwick
barwick@gmx.net

On Sun, 2 Mar 2003, Ian Barwick wrote:

> Date: Sun, 2 Mar 2003 20:02:39 +0100
> From: Ian Barwick <barwick@gmx.net>
> To: Tom Lane <tgl@sss.pgh.pa.us>, Adam Haberlach <adam@newsnipple.com>
> Cc: pgsql-interfaces@postgresql.org
> Subject: Re: [INTERFACES] perl dbd libraries keeping transactions open?
> 
> On Sunday 02 March 2003 18:55, Tom Lane wrote:
> > Adam Haberlach <adam@newsnipple.com> writes:
> > >     It seems that the implementation is for the libs to
> > > do a BEGIN on connect and after every transaction -- is
> > > this normal.  Is there a way to keep it from causing
> > > problems with vaccuums?
> >
> > This is horrible practice for a number of reasons, not only its effect
> > on VACUUM.  Consider what you will get from now(), for example.  A new
> > BEGIN should only be issued when you are ready to issue the first
> > command of the next transaction.

Ah yes, but that begs the question:  what is the first command, because 
DBD::Pg will talk to the DB independently of the application.  So does 
this mean that we issue the begin when a user does a $dbh->tables; 
$dbh->column_info; $dbh->prepare;  $dbh->rollback; $dbh->ping();   What 
this means is that you can have commands other than $sth->execute(); that 
can just begin a tx on you.


> >
> > If that hasn't been fixed yet in the dbd driver, I would recommend
> > fixing it there.
> 

DBD::Pg still has the old autocommit behavour. There is a patch on gborg, 
'http://gborg.postgresql.org/project/dbdpg/bugs/bugupdate.php?398' that 
suposedly fixes this problem; however, it does not answer all of the 
questions that I have as to when to do the BEGIN when in autotx mode, and 
I think it might be better to use PostgreSQL's SET AUTOCOMMIT (if avail).


> The default DBD::Pg behaviour is AutoCommit On, and AFAIK

The default DBI behaviour is AutoCommit On.

> has always been that way. The DBD::Pg driver will only
> begin a transaction if AutoCommit was explicitly set to Off
> or $dbh->begin_work() was executed by the application.

I don't think $dbh->begin_work() is supported by DBD::Pg right now and
will not do things as you expect:
(note:  I have not tested the following code)

$dbh->begin_work();        # begin tx.
eval {    $sth->execute();    $dbh->commit();        # rollback tx and start a new one.

}; if (my $e = $@) {    $dbh->rollback();        #rollback tx and start a new one.
}

# we are now in tx.