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

On 1/14/19 12:20 PM, Dean Rasheed wrote:
> (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.

Yeah, it should have been

if (!(attribute_referenced && reset_stats))

i.e. there's a parenthesis missing. Thanks for noticing this. I guess a
regression test for this would be useful.

> 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.
Hmmm. I'm pretty sure I came to the conclusion it's in fact necessary,
but I might be wrong. Will check.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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