Re: Poor row estimates from planner, stat `most_common_elems` sometimes missing for a text[] column

On 6/5/25 17:42, Mark Frost wrote:
> Is there any good explanation for this behaviour? Preferably we’d like
> some way for proper `most_common_elems` statistics to be collected in
> our production database, in the hope that influences a good query plan
> to always be selected.

most_common_elems has a limited size, and if all the elements have the
same freq, there's nothing we can do.

You could do: alter table test alter column tags set statistics X;

However, X is capped at 10000, which means that the size of
most_common_elems will be less than 100k, and it would probably be
stupid to go beyond that anyway.

It seems that postgres lacks some kind of "n_distinct_elems" for that
kind of case, but let's wait and see what the statistics gurus think.