Re: Estimation error in n_dead_tuples

ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.jp> writes:
> I'm thinking to add "the number of vacuumed tuples" to the message from
> vacuum. The stats collector will subtract the value from n_dead_tuples
> instead of setting it to zero. This is also needed if we want to make
> some kinds of "partial" vacuum methods.

This seems awfully dangerous to me, because then you are operating on
dead reckoning forever: there will be nothing that can correct an
inaccurate rowcount estimate, and in practice that means it will diverge
arbitrarily far from reality :-(, because of the inherent inaccuracies
of the stats system.  I think the risk of that is far worse than the
relatively small (or at least bounded) error arising from tuples not
seen by vacuum.
        regards, tom lane