[GitHub] [arrow] jonkeane commented on pull request #11754: ARROW-14778: [C++] Mean of Decimal should be Double

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jonkeane commented on pull request #11754:
URL: https://github.com/apache/arrow/pull/11754#issuecomment-975940747


   You mention this in the Jira, but could we add in the cast back to decimal proactively? What I'm seeing in postgres (haven't tried others yet) is it "just" blows up the precision on the decimal but does still return a decimal with `AVG`.
   
   If someone has a decimal already it seems odd that it would return a double when a decimal result is possible (it's not quite like division of integers where you have to return something with more precision than an integer there). Even of (for now? until someone complains?) we cast back to the same decimal precision as the input, I think that would be more natural for what operations on decimals like this behave like elsewhere.
   
   ```
   postgres=# create table df (dec decimal(3, 2));
                   insert into df (dec) values (0.01);
                   insert into df (dec) values (0.02);
                   insert into df (dec) values (0.02);
                   insert into df (dec) values (0.02);
   CREATE TABLE
   INSERT 0 1
   INSERT 0 1
   INSERT 0 1
   INSERT 0 1
   postgres=# alter table df add column flt float;
                   insert into df (flt) values (0.01);
                   insert into df (flt) values (0.02);
                   insert into df (flt) values (0.02);
                   insert into df (flt) values (0.02);
   ALTER TABLE
   INSERT 0 1
   INSERT 0 1
   INSERT 0 1
   INSERT 0 1
   postgres=# select pg_typeof(dec), pg_typeof(flt) from df;
    pg_typeof |    pg_typeof     
   -----------+------------------
    numeric   | double precision
    numeric   | double precision
    numeric   | double precision
    numeric   | double precision
    numeric   | double precision
    numeric   | double precision
    numeric   | double precision
    numeric   | double precision
   (8 rows)
   
   postgres=# select AVG(dec), pg_typeof(AVG(dec)), AVG(flt), pg_typeof(AVG(flt)) from df;
             avg           | pg_typeof |  avg   |    pg_typeof     
   ------------------------+-----------+--------+------------------
    0.01750000000000000000 | numeric   | 0.0175 | double precision
   ```


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