Incorrect result for decimal cast #14450
Description
Describe the bug
Converting a decimal 5.20 from (4,2) to (3,2) leads to the value becoming 5.21
Something's gone wrong likely in the fixing of #13492 / apache/arrow-rs#6833?
Or, maybe there's some existing bug in the creation of the decimal array in the first place, and the Arrow bug was hiding it before?
Fyi @alamb @andygrove
To Reproduce
With datafusion-cli built from latest develop, and also from this arrow bump commit a407500:
> select cast(cast('5.20' as decimal(4,2)) as decimal(3,2)), cast(cast(5.20 as decimal(4,2)) as decimal(3,2));
+--------------+--------------+
| Utf8("5.20") | Float64(5.2) |
+--------------+--------------+
| 5.21 | 5.21 |
+--------------+--------------+
1 row(s) fetched.
Elapsed 0.007 seconds.
Expected behavior
With commit 3082ac6807d3bac1dae1e8fb31490d2aa1aec19d (pre- arrow bump):
DataFusion CLI v44.0.0
> select cast(cast('5.20' as decimal(4,2)) as decimal(3,2)), cast(cast(5.20 as decimal(4,2)) as decimal(3,2));
+--------------+--------------+
| Utf8("5.20") | Float64(5.2) |
+--------------+--------------+
| 5.20 | 5.20 |
+--------------+--------------+
1 row(s) fetched.
Elapsed 0.017 seconds.
Additional context
Couple other examples:
> select cast(cast('5.20' as decimal(5,3)) as decimal(4,3)) as a, cast(cast('5.20' as decimal(5,2)) as decimal(3,2)) as b, cast(cast('5.20' as decimal(5,3)) as decimal(4,2)) as c;
+-------+------+------+
| a | b | c |
+-------+------+------+
| 5.201 | 5.21 | 5.20 |
+-------+------+------+
1 row(s) fetched.
> select cast(cast('5.20' as decimal(4,2)) as decimal(2,2));
Arrow error: Invalid argument error: 521 is too large to store in a Decimal128 of precision 2. Max is 99
> select cast(cast('5.20' as decimal(5,3)) as decimal(3,3));
Arrow error: Invalid argument error: 5201 is too large to store in a Decimal128 of precision 3. Max is 999
where does that additional +1 come from?