Re: Merging statistics from children instead of re-sampling everything

On 3/29/21 8:36 PM, Justin Pryzby wrote:
> Thanks for taking a fresh look at this.
>
> As you've written it, this can apply to either/both partitioned or inheritence.
> I imagine when "MERGE" goes away, this should apply only to partitioned tables.
> (Actually, personally I would advocate to consider applying it to *both*, but I
> don't think that's been the tendency over the last 4 years. I wrote here about
> some arguably-gratuitous differences between inheritence and partitioning.
> https://www.postgresql.org/message-id/20180601221428.GU5164@telsasoft.com)
>

Haven't thought about that too much at this point, but I don't see any
reason to not apply it only to both cases. I might be missing something,
but the fact that with declarative partitioning we analyze the children,
while with inheritance we don't, seems a bit strange. Not sure.

>> + if (*mcv_items > default_statistics_target)
>> + n = default_statistics_target;
>
> It should use any non-default stats target of the parent's column
>

Yeah. That's a simplification, the non-PoC code would have to look at
per-column statistics target for the target / all the children, and make
some calculation based on that. I ignored that in the PoC.

>> + * ignore anything but valid leaf relatiins with data, but release
>
> sp: relatiins.
>
>> + elog(WARNING, "stats for %d %d not found",
>> + RelationGetRelid(rels[j]), vacattrstats[i]->attr->attnum);
>
> should be %u %d
>
> This code duplication is undesirable:
>> + /* Log the action if appropriate */
>> + * Determine which columns to analyze
Yeah. Fine for PoC, but needs to be cleaned up in future patch.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company