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GH-33999: [Go] Removed cast from byte[] to string and copied entire string when Value() is called #34450

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GH-33999: [Go] Removed cast from byte[] to string and copied entire string when Value() is called #34450

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7 changes: 5 additions & 2 deletions go/arrow/array/string.go
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Original file line number Diff line number Diff line change
Expand Up @@ -50,8 +50,11 @@ func (a *String) Reset(data arrow.ArrayData) {

// Value returns the slice at index i. This value should not be mutated.
func (a *String) Value(i int) string {
sb := strings.Builder{}

i = i + a.array.data.offset
return a.values[a.offsets[i]:a.offsets[i+1]]
sb.WriteString(a.values[a.offsets[i]:a.offsets[i+1]])
return sb.String()
Comment on lines +53 to +57
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@zeroshade zeroshade Mar 6, 2023 •
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There's no need to use the string builder here, you can just do string([]byte(a.values[a.offsets[i]:a.offsets[i+1]])[:]) or something like:

val := a.values[a.offsets[i]:a.offsets[i+1]]
out := make([]byte, len(val))
copy(out, val)
return *(*string)(unsafe.Pointer(&out))

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Ideally we'd still maintain a method that would let someone retrieve the slice referencing the data to allow consumers to avoid the copy if desired (such as the GetView in the C++ lib)

}

// ValueOffset returns the offset of the value at index i.
Expand Down Expand Up @@ -246,7 +249,7 @@ func (a *LargeString) setData(data *Data) {

if vdata := data.buffers[2]; vdata != nil {
b := vdata.Bytes()
a.values = *(*string)(unsafe.Pointer(&b))
a.values = string(b[:])
Comment on lines 250 to +252
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we shouldn't copy the entire buffer here. Keep the unsafe.Pointer here so we only perform copies in the Value method and only on the values that are requested.

}

if offsets := data.buffers[1]; offsets != nil {
Expand Down