Re: COUNT(*) and index-only scans

Greg Stark <stark@mit.edu> writes:
> Assuming you're in a steady-state situation the amount of all-visible
> blocks will fluctuate from a high just after vacuum to a low just
> before the next vacuum. There are other ways a block can be marked
> all-visible but for the most part I would expect the fraction to go
> steadily down until vacuum comes along and cleans things up.

> So if vacuum tracked the fraction of blocks marked all-visible
> *before* it processed them and the fraction it marked all-visible
> after processing we have an upper and lower bound. If we knew how long
> it's been since vacuum we could interpolate between those, or we could
> just take the mean, or we could take the lower bound as a conservative
> estimate.

I thought of that too, but we don't do the comparable thing for dead
tuple counts, and I am not convinced that we should do it for
visibility.  I'd rather have a simple rule that "it's right immediately
after VACUUM", so that at least trivial cases like read-only tables
work correctly.

>> What I suggest as a first cut for that is: simply derate the visibility fraction as the fraction
>> of the table expected to be scanned gets smaller.

> I think there's a statistically more rigorous way of accomplishing the
> same thing. If you treat the pages we estimate we're going to read as
> a random sample of the population of pages then your expected value is
> the fraction of the overall population that is all-visible but your
> 95th percentile confidence interval will be, uh, a simple formula we
> can compute but I don't recall off-hand.

The problem is precisely that the pages a query is going to read are
likely to *not* be a random sample, but to be correlated with
recently-dirtied pages.

> ... It currently uses all expected values but in many
> cases it would be valuable if the planner knew what the standard
> deviation of those estimates was.

No doubt, but I'm not volunteering to fix that before we can have a
non-toy estimate for index-only scans.
        regards, tom lane