> Mainly 'cause it doesn't know where the dead tuples are till it's
> looked.
At this point I feel very stupid...
> 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.
Ah, now I just feel enlightened! That makes perfect sense. I think I had
been conflating free pages with free space, without understanding what the
difference was. Of course I still don't really understand, but at least I
now _know_ I don't.
Many thanks
Matt