`ScalarValue::try_from_array` holds onto input array for Struct, Lists types

[Dandandan]

Describe the bug

ScalarValue::try_from_array are used in areas such as accumulators.
Currently arrowslice is used for Struct/Union/List types, causing accumulators holding onto the original arrays (rather than a single value), and thus higher memory usage.

To Reproduce

Create an (struct / list type) array of size n and use ScalarValue::try_from_array to convert it into a ScalarValue.

Expected behavior

Value is copied into a new Array or there would be needing efficient way of representing the scalar value.
Memory usage should be corresponding to a single value rather than the full input array.

Additional context

No response

[chenkovsky]

take

[chenkovsky]

Hi, @Dandandan .

it seems that there's no direct way to deep copy array in arrow?
so I tested the following code. it seems that it works.

let a = array.slice(index, 1);

// copy
let data = a.to_data();
let mut copy = MutableArrayData::new(vec![&data], true, data.len());
copy.extend(0, 0, data.len());
let a = make_array(copy.freeze());

Self::Struct(Arc::new(a.as_struct().to_owned()))

But I'm wondering whether it's better to provide a deep copy method for ScalarValue rather than modify try_from_array.
then we can just copy the scalar value when it's needed since deep copy is expensive.