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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