Re: Collect frequency statistics for arrays

On Thu, Mar 1, 2012 at 1:09 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Alexander Korotkov <aekorotkov@gmail.com> writes:
> > On Thu, Mar 1, 2012 at 12:39 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> I am starting to look at this patch now.  I'm wondering exactly why the
> >> decision was made to continue storing btree-style statistics for arrays,
>
> > Probably, btree statistics really does matter for some sort of arrays?
> For
> > example, arrays representing paths in the tree. We could request a
> subtree
> > in a range query on such arrays.
>
> That seems like a pretty narrow, uncommon use-case.  Also, to get
> accurate stats for such queries that way, you'd need really enormous
> histograms.  I doubt that the existing parameters for histogram size
> will permit meaningful estimation of more than the first array entry
> (since we don't make the histogram any larger than we do for a scalar
> column).
>
> The real point here is that the fact that we're storing btree-style
> stats for arrays is an accident, backed into by having added btree
> comparators for arrays plus analyze.c's habit of applying default
> scalar-oriented analysis functions to any type without an explicit
> typanalyze entry.  I don't recall that we ever thought hard about
> it or showed that those stats were worth anything.
>

OK. I don't object to removing btree stats from arrays.
What do you thinks about pg_stats view in this case? Should it combine
values histogram and array length histogram in single column like do for
MCV and MCELEM?

------
With best regards,
Alexander Korotkov.