Re: vacuum, performance, and MVCC - Mailing list pgsql-hackers

Bruce Momjian wrote:
> 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.

Thinking some more, there will need to be a bit to uniquely identify the
head of a CITC.  With that, you could just scan the page tuples looking
for CITC heads, and checking those to see if they are not visible, and
re-using them, rather than doing a full page reorganization where all
free spaces is collected in the middle of the page.  That should limit
the overhead of reuse.

--  Bruce Momjian   bruce@momjian.us EnterpriseDB    http://www.enterprisedb.com
 + If your life is a hard drive, Christ can be your backup. +