Re: vacuum, performance, and MVCC

Zeugswetter Andreas DCP SD wrote:
> 
> > > 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.
> 
> I don't think so. It would probably be sufficient to impose an order on
> the CITC.
> e.g. the oldest tuple version in the CITC is the head. 
> (An idea just in case we can't spare a bit :-) 

Well, if we need to scan the page quickly, having the bit, or a bit
combination that can only be the head, is helpful.  What we usually do
is to combine a SITC bit with another bit that would never be set for
SITC, and that is the head, and you use macros to properly do tests.  We
do this already in a number of cases.

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