Ravi Krishna <srkrishna(at)yahoo(dot)com> writes:
>> VACUUM would not change the state of the dropped columns.
> When does it change?
Never, unless you drop and recreate the table. Removing a dropped
column would change the attnums of following columns, which we
can't support because the tableoid+attnum is the only persistent
identifier of a column.
(From memory, operations like VACUUM FULL and CLUSTER will rewrite
dropped columns with NULLs to reduce their storage impact. But they
don't go away.)
regards, tom lane