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

(Removing Adrien from the CC list, because messages to that address
keep bouncing)

On Sun, 13 Jan 2019 at 00:31, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>
> On 1/10/19 6:09 PM, Dean Rasheed wrote:
> >
> > In the previous discussion around UpdateStatisticsForTypeChange(), the
> > consensus appeared to be that we should just unconditionally drop all
> > extended statistics when ALTER TABLE changes the type of an included
> > column (just as we do for per-column stats), since such a type change
> > can rewrite the data in arbitrary ways, so there's no reason to assume
> > that the old stats are still valid. I think it makes sense to extract
> > that as a separate patch to be committed ahead of these ones, and I'd
> > also argue for back-patching it.
>
> Wasn't the agreement to keep stats that don't include column values
> (functional dependencies and ndistinct coefficients), and reset only
> more complex stats? That's what happens in master and how it's extended
> by the patch for MCV lists and histograms.
>

Ah OK, I misremembered the exact conclusion reached last time. In that
case the logic in UpdateStatisticsForTypeChange() looks wrong:

/*
* If we can leave the statistics as it is, just do minimal cleanup
* and we're done.
*/
if (!attribute_referenced && reset_stats)
{
ReleaseSysCache(oldtup);
return;
}

That should be "|| !reset_stats", or have more parentheses. In fact, I
think that computing attribute_referenced is unnecessary because the
dependency information includes the columns that the stats are for and
ATExecAlterColumnType() uses that, so attribute_referenced will always
be true.

Regards,
Dean