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Re: SET autocommit begins transaction?

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Sean Chittenden <sean(at)chittenden(dot)org>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: SET autocommit begins transaction?
Date: 2002-09-18 22:18:00
Message-ID: 200209182218.g8IMI0D02464@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-bugs
Sean Chittenden wrote:
> > > db.transaction do |dbh|
> > >   db.do('DELETE FROM tbl WHERE id = 5')
> > >   db['AutoCommit'] = true
> > > end
> > > 
> > > Because there wasn't a commit given, that shouldn't actually
> > > delete the rows found, but by tossing that AutoCommit in there, it
> > > should and will generate a nifty warning if AutoCommit sends the
> > > above BEGIN/SET/COMMIT.  -sc
> > 
> > You can't be setting autocommit willy-nilly.  What I was going to
> > suggest is that we allow 'SET autocommit' only at the start of a
> > transaction, and then have it take effect immediately.  If you try
> > autocommit when a transaction is already in progress from a previous
> > statement, we throw an error.
> 
> But that'd result in at least two transactions per connection because
> in my database class wrapper I turn autocommit off.  Under any kind of
> load or performance situations, that's pretty unacceptable.  Granted
> there's nothing that would need to be flushed to disk (hopefully), it
> still strikes me that there would have to be some locking involved and
> that would degrade the performance of the entire system.

You would never see a performance hit.  It doesn't dirty any buffers or
anything.  Heck, a SET with autocommit on is already in its own
transaction.

> If you're throwing an error in the middle of a transaction just
> because of 'SET autocommit', aren't you already making an exception
> and one that degrades the performance of the entire system as a
> result?

I think if we special case autocommit we have to force it to start a
transaction.

> I just saw Tom's post and it seems like something has to give
> someplace...  I'm not a fan of the idea of creating the special case,
> don't get me wrong, but is there a reasonable alternative?  -sc

I am willing to special case autocommit because it is so tied to
transactions anyway.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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Subject: Re: SET autocommit begins transaction?
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