Re: Column storage positions

Alvaro Herrera wrote:
> Bruce Momjian escribi?:
> > Phil Currier wrote:
> > > On 2/21/07, Alvaro Herrera <alvherre(at)commandprompt(dot)com> wrote:
> > > > I'd expect the system being able to reoder the columns to the most
> > > > efficient order possible (performance-wise and padding-saving-wise),
> > > > automatically. When you create a table, sort the columns to the most
> > > > efficient order; ALTER TABLE ADD COLUMN just puts the new columns at the
> > > > end of the tuple; and anything that requires a rewrite of the table
> > > > (ALTER TABLE ... ALTER TYPE for example; would be cool to have CLUSTER
> > > > do it as well; and do it on TRUNCATE also) again recomputes the most
> > > > efficient order.
> > >
> > > That's exactly what I'm proposing. On table creation, the system
> > > chooses an efficient column order for you. The next time an ALTER
> > > TABLE operation forces a rewrite, the system would recompute the
> > > column storage order. I hadn't thought of having CLUSTER also redo
> > > the storage order, but that seems safe since it takes an exclusive
> > > lock on the table. I'm less sure about whether it's safe to do this
> > > during a TRUNCATE.
> >
> > Keep in mind we have a patch in process to reduce the varlena length and
> > reduce alignment requirements, so once that is in, reordering columns
> > will not be as important.
>
> Yes, but the "cache offset" stuff is still significant, so there will be
> some benefit in putting all the fixed-length attributes at the start of
> the tuple, and varlena atts grouped at the end.

Agreed.

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

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