Re: list of extended statistics on psql - Mailing list pgsql-hackers

Hi Tomas,

>>> As for how to deal with this, I can think of about three ways:
>>>
>>> 1) simplify the command to only print information from pg_statistic_ext (so on information about which stats are
builtor sizes)
 
>>>
>>> 2) extend pg_stats_ext with necessary information (e.g. sizes)
>>>
>>> 3) create a new system view, with necessary information (so that pg_stats_ext does not need to be modified)
>>>
>>> 4) add functions returning the necessary information, possibly only for statistics the user can access (similarly
towhat pg_stats_ext does)
 
>>>
>>> Options 2-4 have the obvious disadvantage that this won't work on older releases (we can't add views or functions
there).So I'm leaning towards #1 even if that means we have to remove some of the details. We can consider adding that
fornew releases, though.
 
>>
>>
>> Thanks for the useful advice. I go with option 1).
>> The following query is created by using pg_stats_ext instead of pg_statistic_ext and pg_statistic_ext_data. However,
Iwas confused
 
>> about writing a part of the query for calculating MCV size because
>> there are four columns related to MCV. For example, most_common_vals, most_common_val_nulls, most_common_freqs, and
most_common_base_freqs.
>> Currently, I don't know how to calculate the size of MCV by using the
>> four columns. Thoughts? :-)
> 
> Well, my suggestion was to use pg_statistic_ext, because that lists all statistics, while pg_stats_ext is filtering
statisticsdepending on access privileges. I think that's more appropriate for \dX, the contents should not change
dependingon the user.
 
> 
> Also, let me clarify - with option (1) we'd not show the sizes at all. The size of the formatted statistics may be
verydifferent from the on-disk representation, so I see no point in showing it in \dX.
 
> 
> We might show other stats (e.g. number of MCV items, or the fraction of data represented by the MCV list), but the
usercan inspect pg_stats_ext if needed.
 
> 
> What we might do is to show those stats when a superuser is running this command, but I'm not sure that's a good idea
(orhow difficult would it be to implement).
 


Thanks for clarifying.
I see that your suggestion was to use pg_statistic_ext, not pg_stats_ext.
And we don't need the size of stats.

If that's the case, we also can't get the status of stats since PG12 or later
because we can't use pg_statistic_ext_data, as you know. Therefore, it would be
better to replace the query with the old query that I sent five months ago like this:

# the old query
SELECT
     stxnamespace::pg_catalog.regnamespace AS "Schema",
     stxrelid::pg_catalog.regclass AS "Table",
     stxname AS "Name",
     (SELECT pg_catalog.string_agg(pg_catalog.quote_ident(attname),', ')
      FROM pg_catalog.unnest(stxkeys) s(attnum)
      JOIN pg_catalog.pg_attribute a ON (stxrelid = a.attrelid AND
      a.attnum = s.attnum AND NOT attisdropped)) AS "Columns",
     'd' = any(stxkind) AS "Ndistinct",
     'f' = any(stxkind) AS "Dependencies",
     'm' = any(stxkind) AS "MCV"
FROM pg_catalog.pg_statistic_ext stat
ORDER BY 1,2;

  Schema | Table  |    Name    | Columns | Ndistinct | Dependencies | MCV
--------+--------+------------+---------+-----------+--------------+-----
  public | hoge1  | hoge1_ext  | a, b    | t         | t            | t
  public | hoge_t | hoge_t_ext | a, b    | t         | t            | t
(2 rows)


The above query is so simple so that we would better to use the following query:

# This query works on PG10 or later
SELECT
     es.stxnamespace::pg_catalog.regnamespace::text AS "Schema",
     es.stxname AS "Name",
     pg_catalog.format('%s FROM %s',
         (SELECT pg_catalog.string_agg(pg_catalog.quote_ident(a.attname),', ')
          FROM pg_catalog.unnest(es.stxkeys) s(attnum)
          JOIN pg_catalog.pg_attribute a
          ON (es.stxrelid = a.attrelid
          AND a.attnum = s.attnum
          AND NOT a.attisdropped)),
     es.stxrelid::regclass) AS "Definition",
     CASE WHEN 'd' = any(es.stxkind) THEN 'defined'
     END AS "Ndistinct",
     CASE WHEN 'f' = any(es.stxkind) THEN 'defined'
     END AS "Dependencies",
     CASE WHEN 'm' = any(es.stxkind) THEN 'defined'
     END AS "MCV"
FROM pg_catalog.pg_statistic_ext es
ORDER BY 1, 2;

  Schema |    Name    |    Definition    | Ndistinct | Dependencies | Dependencies
--------+------------+------------------+-----------+--------------+--------------
  public | hoge1_ext  | a, b FROM hoge1  | defined   | defined      | defined
  public | hoge_t_ext | a, b FROM hoge_t | defined   | defined      | defined
(2 rows)


I'm going to create the WIP patch to use the above queriy.
Any comments welcome. :-D

Thanks,
Tatsuro Yamada