Re: DROP COLUMN (was RFC: Restructuring pg_aggregate)

"Christopher Kings-Lynne" <chriskl(at)familyhealth(dot)com(dot)au> writes:
>> No, VACUUM has the same transactional constraints as everyone else
>> (unless you'd like a crash during VACUUM to trash your table...)

> Seriously, you can run VACUUM in a transaction and rollback the movement of
> a tuple on disk? What do you mean by same transactional constraints?

In VACUUM FULL, tuples moved to compact the table aren't good until you
commit. In this hypothetical column-drop-implementing VACUUM, I think
there'd need to be some similar rule --- otherwise it's not clear what
happens to TOASTED data if you crash partway through. (In particular,
if we tried overwriting main tuples in place as Hannu was suggesting,
we'd need a way of being certain the deletion of the corresponding TOAST
rows occurs *before* we overwrite the only reference to them.)

regards, tom lane