Re: More stable query plans via more predictable column statistics

Hi,

On Wed, 2016-03-09 at 11:23 +0100, Shulgin, Oleksandr wrote:
> On Tue, Mar 8, 2016 at 8:16 PM, Alvaro Herrera
> <alvherre@2ndquadrant.com> wrote:
>
>         Also, I can't quite figure out why the "else" now in line 2131
>         is now "else if track_cnt != 0".  What happens if track_cnt is
>         zero?
>         The comment above the "if" block doesn't provide any guidance.
>
>
> It is there only to avoid potentially dividing zero by zero when
> calculating avgcount (which will not be used after that anyway).  I
> agree it deserves a comment.

That definitely deserves a comment. It's not immediately clear why
(track_cnt != 0) would prevent division by zero in the code. The only
way such error could happen is if ndistinct==0, because that's the
actual denominator. Which means this

    ndistinct = ndistinct * sample_cnt

would have to evaluate to 0. But ndistinct==0 can't happen as we're in
the (nonnull_cnt > 0) branch, and that guarantees (standistinct != 0).

Thus the only possibility seems to be (nonnull_cnt==toowide_cnt). Why
not to use this condition instead?