Subject: Re: pg_stats and range statistics
To: Egor Rogov <e.rogov@postgrespro.ru>,
 Soumyadeep Chakraborty <soumyadeep2007@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
References: <b67d8b57-9357-7e82-a2e7-f6ce6eaeec67@postgrespro.ru>
 <55c8ecd4-57d0-5c63-6fe3-1b81e8caea9d@enterprisedb.com>
 <CAE-ML+_69xE=hhw6ZU-BT5h9mR8u8p9BGrikToKPTGthK_VmZw@mail.gmail.com>
 <c178ed2d-d06a-a178-ff9e-f30547ff0e03@postgrespro.ru>
From: Tomas Vondra <tomas.vondra@enterprisedb.com>
Message-ID: <b4c7d719-41c3-4883-463a-6cd4a407b5ee@enterprisedb.com>
Date: Mon, 12 Jul 2021 15:04:08 +0200
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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
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