update == delete + insert?

["Craig A. James" <cjames@xxxxxxxxxxxxxxxx>]

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