Re: statistics for array types - Mailing list pgsql-hackers

On Thu, Aug 20, 2015 at 6:00 PM, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:

Hi,

On 08/11/2015 04:38 PM, Jeff Janes wrote:

When reviewing some recent patches, I decided the statistics gathered
 for arrays had some pre-existing shortcomings.

The main one is that when the arrays contain rare elements there is
no histogram to fall back upon when the MCE array is empty, the way
there is for scalar stats.  So it has to punt completely and resort
to saying that it is 0.5% selectivity without recourse to any data at
all.

The rationale for applying the threshold before things are eligible
for inclusion in the MCE array seems to be that this puts some
theoretical bound on the amount of error we are likely to have in
that element.  But I think it is better to exceed that theoretical
bound than it is to have no data at all.

The attached patch forces there to be at least one element in MCE,
keeping the one element with the highest predicted frequency if the
MCE would otherwise be empty.  Then any other element queried for is
assumed to be no more common than this most common element.

We only really need the frequency, right? So do we really need to keep
the actual MCV element? I.e. most_common_elem_freqs does not have the
same number of values as most_common_elems anyway:

  A list of the frequencies of the most common element values, i.e., the
  fraction of rows containing at least one instance of the given value.
  Two or three additional values follow the per-element frequencies;
  these are the minimum and maximum of the preceding per-element
  frequencies, and optionally the frequency of null elements.
  (Null when most_common_elems is.)

So we might modify it so that it's always defined - either it tracks the
same values as today (when most_common_elems is defined), or the
frequency of the most common element (when most_common_elems is NULL).

I had also considered that.  It requires more changes to make it happen, and it seems to create a more complex contract on what those columns mean, but without giving a corresponding benefit.

 

This way we can keep the current theoretical error-bound on the MCE
frequencies, and if that's not possible we can have at least the new
value without confusing existing code.

But if the frequency of the most common element was grossly wrongly, then whatever value we stick in there is still going to be grossly wrong.  Removing the value associated with it isn't going to stop it from being wrong.  When we do query with the (incorrectly thought) first most common element, either it will find and use the wrong value from slot 1, or it will find nothing and fall back on the same wrong value from slot 3.