Re: Frequent Update Project: Design Overview of HOT

Ühel kenal päeval, R, 2006-11-10 kell 09:06, kirjutas Hannu Krosing:
> Ühel kenal päeval, N, 2006-11-09 kell 18:28, kirjutas Tom Lane:
> > "Simon Riggs" <simon@2ndquadrant.com> writes:
> > > As more UPDATEs take place these tuple chains would grow, making
> > > locating the latest tuple take progressively longer.
> > 
> > This is the part that bothers me --- particularly the random-access
> > nature of the search.  I wonder whether you couldn't do something
> > involving an initial table fill-factor of less than 50%, and having
> > the first updated version living on the same heap page as its parent.
> > Only when the active chain length was more than one (which you
> > hypothesize is rare) would it actually be necessary to do a random
> > access into the overflow table.
> > 
> > More generally, do we need an overflow table at all, rather than having
> > these overflow tuples living in the same file as the root tuples?  As
> > long as there's a bit available to mark a tuple as being this special
> > not-separately-indexed type, you don't need a special location to know
> > what it is.  This might break down in the presence of seqscans though.
> 
> And why do you need to mark it as not-separately-indexed at all ?
> 
> We already cope with missing index pointers in VACUUM and I can't see
> any other reason to have it.

Ok, now I see it - we can't VACUUM a tuple, if next versions of it are
accessible by t_ctid chain only. That is vacuum must not free tuples,
which have t_ctid pointing to a tuple that has not-separately-indexed
bit set. This seems to make vacuum quite complicated, as it has to
examine c_tid chains to detect if it can free a tuple, and what's even
worse, it has to examine these chains backwards.

> What are the advantages of HOT over SITC (other than cool name) ?

still wondering this, is it just the abilty to span multiple pages ?

> Maybe just make HOT an extended SITC which can span pages.
> 
> In case of HOT together with reusing index tuples with DELETED bit set
> we don't actually need copyback, but the same index pointer will follow
> the head of live data automatically, maybe lagging only a small number
> of versions.

> > Actually, you omitted to mention the locking aspects of moving tuples
> > around --- exactly how are you going to make that work without breaking
> > concurrent scans?
> > 
> > > This allows the length of a typical tuple chain to be extremely short in
> > > practice. For a single connection issuing a stream of UPDATEs the chain
> > > length will no more than 1 at any time.
> > 
> > Only if there are no other transactions being held open, which makes
> > this claim a lot weaker.
> > 
> > > HOT can only work in cases where a tuple does not modify one of the
> > > columns defined in an index on the table, and when we do not alter the
> > > row length of the tuple.
> > 
> > Seems like "altering the row length" isn't the issue, it's just "is
> > there room on the page for the new version".  Again, a generous
> > fillfactor would give you more flexibility.
> 
> Maybe they hoped to take very light locks when new chaoin head is copied
> iver the old one in the same-length case.
> 
> >                 http://www.postgresql.org/about/donate
-- 
----------------
Hannu Krosing
Database Architect
Skype Technologies OÜ
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