Re: update == delete + insert?

"Craig A. James" <cjames@modgraph-usa.com> writes:
> 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'scontents are in a file specifically for that column. 

No.  The reason "drop column" is fast is that we make no attempt to
remove the data from existing rows; we only mark the column's entry in
the system catalogs as deleted.  "add column" is only fast if you are
adding a column with no default (a/k/a default NULL).  In that case
likewise we don't have to modify existing rows; the desired behavior
falls out from the fact that the tuple access routines return NULL if
asked to fetch a column beyond those existing in a particular tuple.

You can read about the storage layout in
http://developer.postgresql.org/docs/postgres/storage.html

            regards, tom lane