Re: vacuum, performance, and MVCC

Martijn van Oosterhout wrote:
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> On Mon, Jun 26, 2006 at 10:50:26AM -0400, Bruce Momjian wrote:
> > > > 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?
> > >
> > > It is probably better to always return the pointer to the head of CITC
> > > chain (the one an index points to) and do extra visibility checks and
> > > chain-following on each access. This would keep the change internal to
> > > tuple fetching functions.
> >
> > So index_getnext() traverses the chain and returns one member per call.
> > Makes sense. Just realize you are in a single index entry returning
> > multiple tuples. We will need some record keeping to track that.
>
> Yes, and for index_getmulti (which doesn't visit the heap at all) we'll
> have to change all the users of that (which aren't many, I suppose).
> It's probably worth making a utility function to expand them.
>
> I'm still confused where bitmap index scan fit into all of this. Is
> preserving the sequential scan aspect of these a goal with this new
> setup?

No. I was just pointing out that if you get to the tuple via an index,
you get handed the head of the SITC via the index tuple, but if you are
doing a sequential scan, you don't get it, so you have to find it, or
any other non-visible SITC header.

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

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