Re: Vacuum Daemon

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.

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