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Re: Vacuum Daemon

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "J(dot) R(dot) Nield" <jrnield(at)usol(dot)com>,"Matthew T(dot) O'Connor" <matthew(at)zeut(dot)net>,PostgreSQL Hacker <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Vacuum Daemon
Date: 2002-06-30 02:12:41
Message-ID: 200206300212.g5U2CfP22909@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane wrote:
> "J. R. Nield" <jrnield(at)usol(dot)com> writes:
> >> I do not think that is the case; and anyway we've pretty much rejected
> >> Vadim's notion of going to an Oracle-style UNDO buffer.
> 
> > Could someone point me to this discussion, or summarize what the problem
> > was?
> 
> I'm too lazy to dig through the archives at the moment, but the main
> points were (a) a finite-size UNDO buffer chokes on large transactions
> and (b) the Oracle approach requires live transaction processing to
> do the cleanup work that our approach can push off to hopefully-not-
> time-critical vacuum processing.
> 
> UNDO per se doesn't eliminate VACUUM anyhow; it only reclaims space
> from tuples written by aborted transactions.  If you want to get rid
> of VACUUM then you need another way to get rid of the old versions of
> perfectly good committed tuples that are obsoleted by updates from
> later transactions.  That essentially means you need an overwriting
> storage manager, which is a concept that doesn't mix well with MVCC.
> 
> Oracle found a solution to that conundrum, but it's really not obvious
> to me that their solution is better than ours.  Also, they have
> patents that we'd probably run afoul of if we try to imitate their
> approach too closely.

Don't forget reclaiming space from transactions that delete tuples.
UNDO doesn't help there either.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026



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