Re: vacuum, performance, and MVCC

Ühel kenal päeval, E, 2006-06-26 kell 23:08, kirjutas Bruce Momjian:
> Jim C. Nasby wrote:
> > On Mon, Jun 26, 2006 at 02:32:59PM -0400, Bruce Momjian wrote:
> > >
> > > It is certainly possible to do what you are suggesting, that is have two
> > > index entries point to same chain head, and have the index access
> > > routines figure out if the index qualifications still hold, but that
> > > seems like a lot of overhead.

I think Jim meant not 2 pointing to the same head, but 2 pointing into
the same chain. Say we have table with (id serial, ts timestamp) where
ts changes at each update and id does not.

So after 3 updates on one page we have one CITC/ITPC head with pointers
from both indexes and two follow-up tuples with pointers from only ts
index.

The problem with this setup is, that we can't reuse any of those
follow-up tuples without index cleanup.

> > > Also, once there is only one visible row in the chain, removing old
> > > index entries seems quite complex because you have to have vacuum keep
> > > the qualifications of each row to figure out which index tuple is the
> > > valid one (seems messy).
> >
> > Perhaps my point got lost... in the case where no index keys change
> > during an update, SITC seems superior in every way to my proposal. My
> > idea (let's call it Index Tuple Page Consolidation, ITPC) would be
> > beneficial to UPDATEs that modify one or more index keys but still put
> > the tuple on the same page. Where SITC would be most useful for tables
> > that have a very heavy update rate and very few indexes, ITPC would
> > benefit tables that have more indexes on them; where presumably it's
> > much more likely for UPDATEs to change at least one index key (which
> > means SITC goes out the window, if I understand it correctly). If I'm
> > missing something and SITC can in fact deal with some index keys
> > changing during an UPDATE, then I see no reason for ITPC.
>
> I understood what you had said. The question is whether we want to get
> that complex with this feature, and if there are enough use cases
> (UPDATE with index keys changing) to warrant it.

I'd like to think that most heavily-updated tables avoid that, but there
may be still cases where this is needed.

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