On Wed, 2002-07-03 at 14:32, Rod Taylor wrote:
> > > It should also be noted that an ALTER TABLE / SET TYPE implemented with
> > > the above idea with run into the 2x diskspace issue as well as take
> > > quite a while to process.
> >
> > I think that if the 'SET TYPE' operation is ever to be rollback-able, it
> > will need to use 2x diskspace. If it's overwritten in place, there's no
> > chance of fallback... I think that a DBA would choose to use the command
> > knowing full well what it requires? Better than not offering them the
> > choice at all!
>
> True, but if we did the multi-version thing in pg_attribute we may be
> able to coerce to the right type on the way out making it a high speed
> change.
If I understand you right, i.e. you want to do the conversion at each
select(), then the change is high speed but all subsequent queries using
it will pay a a speed penalty, not to mention added complexity of the
whole thing.
I don't think that making changes quick autweights added slowness and
complexity - changes are meant to be slow ;)
The real-life analogue to the proposed scenario would be adding one
extra wheel next to each existing one in a car in order to make it
possible to change tyres while driving - while certainly possible nobody
actually does it.
---------------
Hannu
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