Re: multivariate statistics (v19)

On 12/12/16 22:50, Tomas Vondra wrote:
>> +<programlisting>
>> +SELECT pg_mv_stats_dependencies_show(stadeps)
>> + FROM pg_mv_statistic WHERE staname = 's1';
>> +
>> + pg_mv_stats_dependencies_show
>> +-------------------------------
>> + (1) => 2, (2) => 1
>> +(1 row)
>> +</programlisting>
>>
>> Couldn't this somehow show actual column names, instead of attribute
>> numbers?
>>
>
> Yeah, I was thinking about that too. The trouble is that's table-level
> metadata, so we don't have that kind of info serialized within the data
> type (e.g. because it would not handle column renames etc.).
>
> It might be possible to explicitly pass the table OID as a parameter of
> the function, but it seemed a bit ugly to me.

I think it makes sense to have such function, this is not out function
so I think it's ok for it to have the oid as input, especially since in
the use-case shown above you can use starelid easily.

>
> FWIW, as I wrote in this thread, the place where this patch series needs
> feedback most desperately is integration into the optimizer. Currently
> all the magic happens in clausesel.c and does not leave it.I think it
> would be good to move some of that (particularly the choice of
> statistics to apply) to an earlier stage, and store the information
> within the plan tree itself, so that it's available outside clausesel.c
> (e.g. for EXPLAIN - showing which stats were picked seems useful).
>
> I was thinking it might work similarly to the foreign key estimation
> patch (100340e2). It might even be more efficient, as the current code
> may end repeating the selection of statistics multiple times. But
> enriching the plan tree turned out to be way more invasive than I'm
> comfortable with (but maybe that'd be OK).
>

In theory it seems like possibly reasonable approach to me, mainly
because mv statistics are user defined objects. I guess we'd have to see
at least some PoC to see how invasive it is. But I ultimately think that
feedback from a committer who is more familiar with planner is needed here.

--
Petr Jelinek http://www.2ndQuadrant.com/
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