Re: vacuum, performance, and MVCC

Bruce Momjian wrote:
> Martijn van Oosterhout wrote:
> -- Start of PGP signed section.
> > On Mon, Jun 26, 2006 at 07:17:31AM -0400, Bruce Momjian wrote:
> > > Correct! We use the same pointers used by normal UPDATEs, except we set
> > > a bit on the old tuple indicating it is a single-index tuple, and we
> > > don't create index entries for the new tuple. Index scan routines will
> > > need to be taught about the new chains, but because only one tuple in
> > > the chain is visible to a single backend, the callers should not need to
> > > be modified.
> >
> > I suppose we would also change the index_getmulti() function to return
> > a set of ctids plus flags so the caller knows to follow the chains,
> > right? And for bitmap index scans you would only remember the page in
> > the case of such a tuple, since you can't be sure the exact ctid you've
> > got is the one you want.
> >
> > Seems doable.
>
> Yes, it just is an issue of where you want to add the complexity ---
> scan entire page when no free space, or only an UPDATE.

Oh, and because you want to do this when doing an update via sequential
scan as well as an index scan, I am thinking you might need to do the
per-page method because you might not have even seen the head of the
chain yet. With an index scan, finding the head is easy, but for a
sequential scan, it seems more difficult, and we don't have any free
space in the tail of the chain to maintain a pointer to the head.

--
Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

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