Re: ALTER TABLE DROP COLUMN

On Mon, 9 Oct 2000, Bruce Momjian wrote:

> > Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > >> Basically, move the first 100 rows to the end of the table file, then take
> > >> 100 and write it to position 0, 101 to position 1, etc ... that way, at
> > >> max, you are using ( tuple * 100 ) bytes of disk space, vs 2x the table
> > >> size ... either method is going to lock the file for a period of time, but
> > >> one is much more friendly as far as disk space is concerned *plus*, if RAM
> > >> is available for this, it might even be something that the backend could
> > >> use up to -S blocks of RAM to do it off disk? If I set -S to 64meg, and
> > >> the table is 24Meg in size, it could do it all in memory?
> >
> > > Yes, I liked that too.
> >
> > What happens if you crash partway through?
> >
> > I don't think it's possible to build a crash-robust rewriting ALTER
> > process that doesn't use 2X disk space: you must have all the old tuples
> > AND all the new tuples down on disk simultaneously just before you
> > commit. The only way around 2X disk space is to adopt some logical
> > renumbering approach to the columns, so that you can pretend the dropped
> > column isn't there anymore when it really still is.
>
> Yes, I liked the 2X disk space, and making the new tuples visible all at
> once at the end.

man, are you ever wishy-washy on this issue, aren't you? :) you like not
using 2x, you like using 2x ... :)