"Craig A. James" <[EMAIL PROTECTED]> 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's contents 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

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