Robert Haas <robertmhaas@gmail.com> wrote:
> I think we should really consider replacing reltuples with
> reltupledensity at some point. I continue to be afraid that using
> a decaying average in this case is going to end up overweighting
> the values from some portion of the table that's getting scanned
> repeatedly, at the expense of other portions of the table that are
> not getting scanned at all. Now, perhaps experience will prove
> that's not a problem. But storing relpages and reltupledensity
> separately would give us more flexibility, because we could feel
> free to bump relpages even when we're not sure what to do about
> reltupledensity. That would help Florian's problem quite a lot,
> even if we did nothing else.
Given how trivial it would be to adjust reltuples to keep its ratio
to relpages about the same when we don't have a new "hard" number,
but some evidence that we should fudge our previous value, I don't
see where this change buys us much. It seems to mostly obfuscate
the fact that we're changing our assumption about how many tuples we
have. I would rather that we did that explicitly with code comments
about why it's justified than to slip it in the way you suggest.
-Kevin
