update == delete + insert?

I've seen it said here several times that "update == delete + insert". On the other hand, I've noticed that "alter table [add|drop] column ..." is remarkably fast, even for very large tables, which leads me to wonder whether each column's contents are in a file specifically for that column.

My question: Suppose I have a very "wide" set of data, say 100 columns, and one of those columns will be updated often, but the others are fairly static. I have two choices:

Design 1:
create table a (
id integer,
frequently_updated integer);

create table b(
id integer,
infrequently_updated_1 integer,
infrequently_updated_2 integer,
infrequently_updated_3 integer,
... etc.
infrequently_updated_99 integer);

Design 2:
create table c(
id integer,
frequently_updated integer,
infrequently_updated_1 integer,
infrequently_updated_2 integer,
infrequently_updated_3 integer,
... etc.
infrequently_updated_99 integer);

If "update == delete + insert" is strictly true, then "Design 2" would be poor since 99 columns would be moved around with each update. But if columns are actually stored in separate files, the Designs 1 and 2 would be essentially equivalent when it comes to vacuuming.

Thanks,
Craig