Re: t_self as system column

Excerpts from Robert Haas's message of mar jul 06 17:24:21 -0400 2010:
> On Tue, Jul 6, 2010 at 5:18 PM, Andrew Dunstan <andrew(at)dunslane(dot)net> wrote:

> > In any case, having a mutable logical column
> > position is the feature that's been most requested.
>
> I think that's true. But the physical storage position would give us
> a performance benefit, by allowing us to try to avoid useless
> alignment padding.

That's true too. I intend to look at both problems simultaneously, i.e.
decoupling the current attnum in three columns as previously discussed;
as Tom says, I think it'll end up being less work than attacking them
separately. However, I will not attempt to include optimizations such
as avoiding padding, in the first patch, just the possibility that it is
added later.