hcrosse opened a new issue, #1497:
URL: https://github.com/apache/arrow-go/issues/1497
### Describe the bug, including details regarding any error messages,
version, and platform.
`arrow/scalar` has no scalar types for `decimal32` or `decimal64`.
`GetScalar` returns an error for valid values but panics on null slots. Null
dictionary decoding and casting a null scalar with `Scalar.CastTo` also panic.
I reproduced the following `decimal32` cases on main at `32947b25` and on
v18.8.0, on macOS arm64 with Go 1.26.10.
**Reproduction**
Save this as `main.go` in a new subdirectory of an arrow-go checkout at
`32947b259414a77ae6347d3d29bd69ec00b9d560`, then run `go run . valid`, `go run
. getscalar`, `go run . encoded` and `go run . castto` from that directory:
```go
package main
import (
"fmt"
"os"
"github.com/apache/arrow-go/v18/arrow"
"github.com/apache/arrow-go/v18/arrow/array"
"github.com/apache/arrow-go/v18/arrow/memory"
"github.com/apache/arrow-go/v18/arrow/scalar"
)
func main() {
d32 := &arrow.Decimal32Type{Precision: 5, Scale: 2}
switch os.Args[1] {
case "valid":
b := array.NewDecimal32Builder(memory.DefaultAllocator, d32)
b.Append(12345)
fmt.Println(scalar.GetScalar(b.NewArray(), 0))
case "getscalar":
arr := array.MakeArrayOfNull(memory.DefaultAllocator, d32, 1)
fmt.Println(scalar.GetScalar(arr, 0))
case "encoded":
dict :=
scalar.NewNullDictScalar(&arrow.DictionaryType{IndexType:
arrow.PrimitiveTypes.Int8, ValueType: d32})
fmt.Println(dict.GetEncodedValue())
case "castto":
fmt.Println(scalar.MakeNullScalar(arrow.PrimitiveTypes.Int32).CastTo(d32))
}
}
```
| Command | Main and v18.8.0 | Expected |
| --- | --- | --- |
| `valid` | `<nil> cannot create scalar from array of type decimal32(5, 2)`
| `123.45 <nil>` |
| `getscalar`, `encoded`, `castto` | nil pointer panic | `null <nil>` |
**Cause**
`GetScalar` has no cases for these decimal types, and their entries in the
null constructor table are nil. `MakeNullScalar` calls those entries, including
through the null branch of `Dictionary.GetEncodedValue`:
https://github.com/apache/arrow-go/blob/32947b259414a77ae6347d3d29bd69ec00b9d560/arrow/scalar/scalar.go#L489-L491
https://github.com/apache/arrow-go/blob/32947b259414a77ae6347d3d29bd69ec00b9d560/arrow/scalar/scalar.go#L586-L610
https://github.com/apache/arrow-go/blob/32947b259414a77ae6347d3d29bd69ec00b9d560/arrow/scalar/nested.go#L522-L528
C++ has
[`Decimal32Scalar`](https://github.com/apache/arrow/blob/1ebb9379b2f635755e773b79e88cb99852c70455/cpp/src/arrow/scalar.h#L592-L594)
and
[`Decimal64Scalar`](https://github.com/apache/arrow/blob/1ebb9379b2f635755e773b79e88cb99852c70455/cpp/src/arrow/scalar.h#L596-L598).
I have a patch with tests and can open a PR.
### Component(s)
Other
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