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

On 21/1/2022 01:25, Tomas Vondra wrote:
> But I don't have a very good idea what to do about statistics that we
> can't really merge. For some types of statistics it's rather tricky to
> reasonably merge the results - ndistinct is a simple example, although
> we could work around that by building and merging hyperloglog counters.
I think, as a first step on this way we can reduce a number of pulled 
tuples. We don't really needed to pull all tuples from a remote server. 
To construct a reservoir, we can pull only a tuple sample. Reservoir 
method needs only a few arguments to return a sample like you read 
tuples locally. Also, to get such parts of samples asynchronously, we 
can get size of each partition on a preliminary step of analysis.
In my opinion, even this solution can reduce heaviness of a problem 
drastically.

-- 
regards,
Andrey Lepikhov
Postgres Professional