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Re: [Bizgres-general] WAL bypass for INSERT, UPDATE and

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andrew Dunstan <andrew(at)dunslane(dot)net>, simon(at)2ndquadrant(dot)com, kleptog(at)svana(dot)org, gsstark(at)mit(dot)edu, pg(at)rbt(dot)ca, zhouqq(at)cs(dot)toronto(dot)edu, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [Bizgres-general] WAL bypass for INSERT, UPDATE and
Date: 2006-01-03 21:20:47
Message-ID: 200601032120.k03LKl609990@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Jim C. Nasby wrote:
> > > I don't think it should (which implies that EXCLUSIVE is a bad name).
> > 
> > Agreed, EXCLUSIVE was used to mean an _exclusive_ writer.  The new words
> > I proposed were PRESERVE or STABLE.
> 
> This seems to seriously limit the usefulness, though. You'll only want
> to use EXCLUSIVE/PRESERVE/STABLE when you've got a specific set of DML
> to do, that you know you can recover from. But if at the same time some
> other part of the system could be doing what it thinks will be ACID DML
> to that same table, you're now in trouble.
> 
> At a minimum that would need to be clearly spelled out in the docs. I
> think it also makes a very strong use-case for exposing table-level
> shared locks as well, since that would at least allow other backends to
> continue reading from the table.

We would be creating a new lock type for this.

> Idealistically, if EXCLUSIVE/PRESERVE/STABLE does it's thing by only
> appending new pages, it would be nice if other backends could continue
> performing updates at the same time, assuming there's free space
> available elsewhere within the table (and that you'd be able to recover
> those logged changes regardless of the non-logged operations). But
> that's a pretty lofty goal...

"Idealistically", yep.  It would be great if we could put a helmet on
and the computer would read your mind.  :-)

Basically meaning your idea of update while EXCLUSIVE/PRESERVE/STABLE is
happening is never going to be implemented because it is just too hard
to do, and too prone to error.

-- 
  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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