Re: pg_stats and range statistics

On 7/12/21 1:10 PM, Egor Rogov wrote:
> Hi,
>
> thanks for the review and corrections.
>
> On 11.07.2021 21:54, Soumyadeep Chakraborty wrote:
>> Hello,
>>
>> This should have been added with [1].
>>
>> Excerpt from the documentation:
>> "pg_stats is also designed to present the information in a more readable
>> format than the underlying catalog — at the cost that its schema must
>> be extended whenever new slot types are defined for pg_statistic." [2]
>>
>> So, I added a reminder in pg_statistic.h.
>
> Good point.
>
>
>> Attached is v2 of this patch with some cosmetic changes.
>
> I wonder why "TODO: catalog version bump"? This patch doesn't change
> catalog structure, or I miss something?
>

It changes system_views.sql, which is catalog change, as it redefines
the pg_stats system view (it adds 3 more columns). So it changes what
you get after initdb, hence catversion has to be bumped.

>
>> Renamed the columns a
>> bit and updated the docs to be a bit more descriptive.
>> (range_length_empty_frac -> empty_range_frac, range_bounds_histogram ->
>> range_bounds_histograms)
>
> I intended to make the same prefix ("range_") for all columns concerned
> with range types, although I'm fine with the proposed naming.
>

Yeah, I'd vote to change empty_range_frac -> range_empty_frac.

>
>> One question:
>>
>> We do have the option of representing the histogram of lower bounds
>> separately
>> from the histogram of upper bounds, as two separate view columns.
>> Don't know if
>> there is much utility though and there is a fair bit of added
>> complexity: see
>> below. Thoughts?
>
> I thought about it too, and decided not to transform the underlying data
> structure. As far as I can see, pg_stats never employed such
> transformations. For example, STATISTIC_KIND_DECHIST is an array
> containing the histogram followed by the average in its last element. It
> is shown in pg_stats.elem_count_histogram as is, although it arguably
> may be splitted into two fields. All in all, I believe pg_stats's job is
> to "unpack" stavalues and stanumbers into meaningful fields, and not to
> try to go deeper than that.
>

Not firm opinion, but the pg_stats is meant to be easier to
read/understand for humans. So far the transformation were simple
because all the data was fairly simple, but the range stuff may need
more complex transformation.

For example we do quite a bit more in pg_stats_ext views, because it
deals with multi-column stats.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company