Re: [HACKERS] PATCH: multivariate histograms and MCV lists

On 26 March 2018 at 14:08, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> On 03/26/2018 12:31 PM, Dean Rasheed wrote:
>> A wider concern I have is that I think this function is trying to be
>> too clever by only resetting selected stats. IMO it should just reset
>> all stats unconditionally when the column type changes, which would
>> be consistent with what we do for regular stats.
>>
> The argument a year ago was that it's more plausible that the semantics
> remains the same. I think the question is how the type change affects
> precision - had the type change in the opposite direction (int to real)
> there would be no problem, because both ndistinct and dependencies would
> produce the same statistics.
>
> In my experience people are far more likely to change data types in a
> way that preserves precision, so I think the current behavior is OK.

Hmm, I don't really buy that argument. Altering a column's type allows
the data in it to be rewritten in arbitrary ways, and I don't think we
should presume that the statistics will still be valid just because
the user *probably* won't do something that changes the data much.

Regards,
Dean