Re: Bug in VACUUM FULL ?

Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> Breaking the chain up into pieces seems weird. It seems like it's obviously
> bogus and only works because we're sure the tuples are dead anyways so it
> doesn't really matter what we do with them.

Yup, exactly. If we wanted to be tense about this we'd try to get rid
of the nominally RECENTLY_DEAD tuples that precede any DEAD tuple in the
chain. However, I concur with Simon to the extent that I don't want to
do any more work to fix this bug than necessary, and trying to recognize
such tuples seems like a lot more work than necessary.

Also, we know this case works because it already is working: in the
situation where VACUUM happens to visit and remove the DEAD tuple(s)
before reaching the RECENTLY_DEAD tuples that link forward to them,
it treats the RECENTLY_DEAD tuples as a disconnected chain and moves
them as-is. I saw tons of this in the traces I was making today, and
it doesn't seem to create any bad effects. (My attention was drawn to
it because I saw move_chain_tuple being used to move single-member
chains, which looks impossible when you first look at the code --- the
is-it-a-chain test seems to ensure that we can link either forward or
backward. But not so if t_ctid points to an already-removed tuple.)

regards, tom lane