"Matt Clark" <matt@ymogen.net> writes:
> OK, I had thought (wrongly it seems, as usual, but this is how we learn!)
> that a plain VACUUM did not incur a read of all pages. I still don't
> understand *why* it does, but I'll take your word for it.
Mainly 'cause it doesn't know where the dead tuples are till it's
looked. Also, VACUUM is the data collector for the free space map,
and so it is also charged with finding out how much free space exists
on every page.
regards, tom lane