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

[GitBox <gi...@apache.org>]

lidavidm commented on pull request #11754:
URL: https://github.com/apache/arrow/pull/11754#issuecomment-975944295


   Just for reference it seems DuckDB actually outputs a double:
   
   ```
   D create table df (dec decimal(3, 2), flt float);
   D insert info df (dec, flt) values (0.01, 0.01);
   Error: Parser Error: syntax error at or near "info"
   LINE 1: insert info df (dec, flt) values (0.01, 0.01);
                  ^
   D insert into df (dec, flt) values (0.01, 0.01);
   D insert into df (dec, flt) values (0.02, 0.02);
   D insert into df (dec, flt) values (0.02, 0.02);
   D insert into df (dec, flt) values (0.02, 0.02);
   D select typeof(dec), typeof(flt) from df;
   ┌──────────────┬─────────────┐
   │ typeof(dec)  │ typeof(flt) │
   ├──────────────┼─────────────┤
   │ DECIMAL(3,2) │ FLOAT       │
   │ DECIMAL(3,2) │ FLOAT       │
   │ DECIMAL(3,2) │ FLOAT       │
   │ DECIMAL(3,2) │ FLOAT       │
   └──────────────┴─────────────┘
   D select avg(dec), avg(flt), typeof(avg(dec)), typeof(avg(flt)) from df;
   ┌──────────┬─────────────────────┬──────────────────┬──────────────────┐
   │ avg(dec) │      avg(flt)       │ typeof(avg(dec)) │ typeof(avg(flt)) │
   ├──────────┼─────────────────────┼──────────────────┼──────────────────┤
   │ 0.0175   │ 0.01749999960884452 │ DOUBLE           │ DOUBLE           │
   └──────────┴─────────────────────┴──────────────────┴──────────────────┘
   ```
   
   We can cast back to Decimal, but how should we round/truncate the result?


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