I wrote:
> Hmm, is that actually true? There's no more reason to think a tuple
> in a temp table is old enough to be visible to all other sessions
> than one in any other table. It could be all right if we had a
> special-case rule for setting all-visible in temp tables. Which
> indeed I thought we had, but I can't find any evidence of that in
> vacuumlazy.c, nor did a trawl of the commit log turn up anything
> promising. Am I just looking in the wrong place?
Ah, never mind that --- I must be looking in the wrong place.
Direct experimentation proves that VACUUM will set all-visible bits
for temp tables even in the presence of concurrent transactions.
regards, tom lane