Re: MCV lists for highly skewed distributions

On 03/17/2018 08:32 PM, Dean Rasheed wrote:
> On 17 March 2018 at 18:40, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
>> Currently, analyze_mcv_list only checks if the frequency of the
>> current item is significantly higher than the non-MCV selectivity.
>> My question is if it shouldn't also consider if removing the item
>> from MCV would not increase the non-MCV selectivity too much.
>>
> 
> Oh, I see what you're saying. In theory, each MCV item we remove is 
> not significantly more common than the non-MCV items at that point,
> so removing it shouldn't significantly increase the non-MCV
> selectivity. It's possible the cumulative effect of removing multiple
> items might start to add up, but I think it would necessarily be a
> slow effect, and I think it would keep getting slower and slower as
> more items are removed -- isn't this equivalent to constructing a
> sequence of numbers where each number is a little greater than the
> average of all the preceding numbers, and ends up virtually
> flat-lining.
> 

Yes, I think you're right. Another thing that occurred to me is that
we're pretty much guaranteed to misestimate things at the tail end, and
in my experience under-estimates have far worse consequences.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services