Date: Wed, 20 Nov 2002 17:00:59 -0700 (MST)
From: "scott.marlowe" <scott.marlowe@ihs.com>
To: Brian Hirt <bhirt@mobygames.com>
Cc: Medi Montaseri <medi.montaseri@intransa.com>,
	Neil Conway <neilc@samurai.com>, Francisco Reyes <lists@natserv.com>,
	pgsql General List <pgsql-general@postgresql.org>
Subject: Re: Closing inactive connections OR user connections limits
In-Reply-To: <1037834219.5828.17.camel@tex.mobygames.com>
Message-ID: <Pine.LNX.4.33.0211201659560.21699-100000@css120.ihs.com>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII

Reread the original message I was replying to.  He was worried that if I 
was to delete rows then roll back, that a vacuum running while I had 
deleted the tuples would prevent the rollback.

I.e. I was proving that you COULD roll back a transaction that had a 
vacuum occur during it.

On 20 Nov 2002, Brian Hirt wrote:

> Scott,
> 
> You did a rollback on the transaction, so the delete was never commited.
> I hope you aren't expecting vacuum to delete uncommited rows from the
> database, are you?
> 
> --brian
> 
> On Wed, 2002-11-20 at 16:12, scott.marlowe wrote:
> > On Wed, 20 Nov 2002, Medi Montaseri wrote:
> > 
> > > Its my understanding that vacuum actually removes tuples that have been 
> > > updated or deleted.
> > > Sort of like emptying your trash .... whence a tuple has been removed, 
> > > no rollback can set the
> > > state back. If you have logically removed a tuple (not vacuumed yet), 
> > > then one can rollback,
> > > but if you vacuum then you can not rollback.
> > > 
> > > Now suppose transaction A decides to delete some tuples, a vacuum job 
> > > comes along and
> > > deletes things (in parallel), trans A decides to rollback....engines who 
> > > support parallel
> > > vacuum-ing and transactions such as PG 7.2 better have a way of 
> > > protecting themselves
> > > against this....
> > > 
> > > Correct me if ...
> > 
> > Yes, you are wrong.  Postgresql's vacuuming does NOT free tuples that are 
> > still in a transaction, hence a full vacuum will hand waiting for the 
> > transaction to complete or roll back.  A normal 7.2 vacuum will simply 
> > skip the in transaction tuples.
> > 
> > For proof, try this:  (Note A> and B> are used to represent two different 
> > sessions)
> > 
> > A> create table test (a text, id int);
> > A> insert into test (a,id) values ('abc',123);
> > A> begin;
> > A> delete from test where id=123;
> > B> vacuum;
> > A> rollback;
> > A> select * from test;
> > a  |  id
> > --------
> > abc| 123
> > 
> > Still there.
> > 
> > 
> > > 
> > > Neil Conway wrote:
> > > 
> > > >Medi Montaseri <medi.montaseri@intransa.com> writes:
> > > >  
> > > >
> > > >>I think from the data integrity point of view, vacuum is more
> > > >>important than vacuum full.
> > > >>    
> > > >>
> > > >
> > > >Why would VACUUM have any effect on data integrity, either positive or
> > > >negative?
> > > >
> > > >Cheers,
> > > >
> > > >Neil
> > > >
> > > >  
> > > >
> > > 
> > > 
> > > 
> > > 
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