Re: Changing Column Order (Was Re: MySQL vs PostgreSQL.)

Jan Wieck wrote:
> Bruce Momjian wrote:
> > 
> > Alessio Bragadini wrote:
> > > On Sat, 2002-10-12 at 11:37, Gavin Sherry wrote:
> > >
> > > > I cannot think of any reason why changing column order should be
> > > > implemented in Postgres. Seems like a waste of time/more code bloat for
> > > > something which is strictly asthetic.
> > > >
> > > > Regardless, I do have collegues/clients who ask when such a feature will
> > > > be implemented. Why is this useful?
> > >
> > > Has column ordering any effect on the physical tuple disposition? I've
> > > heard discussions about keeping fixed-size fields at the beginning of
> > > the tuple and similar.
> > >
> > > Sorry for the lame question. :-)
> > 
> > Yes, column ordering matches physical column ordering in the file, and
> > yes, there is a small penalty for accessing any columns after the first
> > variable-length column (pg_type.typlen < 0). CHAR() used to be a fixed
> > length column, but with TOAST (large offline storage) it became variable
> > length too.  I don't think there is much of a performance hit, though.
> 
> When was char() fixed size? We had fixed size things like char, char2,
> char4 ... char16. But char() is internally bpchar() and has allways been
> variable-length.

char() was fixed size only in that you could cache the column offsets
for char() becuase it was always the same width on disk before TOAST.

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