tustvold opened a new issue, #1799:
URL: https://github.com/apache/arrow-rs/issues/1799
**TLDR**
Make ArrayData layout explicit so that we can eventually push offsets down
into the underlying buffers/bitmaps, instead of tracking them as a top-level
concept which has proven to be rather error prone.
**Is your feature request related to a problem or challenge? Please describe
what you are trying to do.**
Currently `ArrayData` is defined as follows.
```
pub struct ArrayData {
/// The data type for this array data
data_type: DataType,
/// The number of elements in this array data
len: usize,
/// The number of null elements in this array data
null_count: usize,
/// The offset into this array data, in number of items
offset: usize,
/// The buffers for this array data. Note that depending on the array
types, this
/// could hold different kinds of buffers (e.g., value buffer, value
offset buffer)
/// at different positions.
buffers: Vec<Buffer>,
/// The child(ren) of this array. Only non-empty for nested types,
currently
/// `ListArray` and `StructArray`.
child_data: Vec<ArrayData>,
/// The null bitmap. A `None` value for this indicates all values are
non-null in
/// this array.
null_bitmap: Option<Bitmap>,
}
```
This is simple, but has a couple of caveats:
* It isn't clear what is present for specific layout types
* There is no clear path to storing `BooleanArray` as `BitMap` vs `Buffer`,
which would allow removing `offset`
* Vec allocations for one or two elements (the C++ implementation inlines
these)
* There is potential for accidentally interpreting a buffer incorrectly
**Describe the solution you'd like**
Introduce a new `ArrayDataLayout` enumeration:
```
pub enum ArrayDataLayout {
Boolean { values: Buffer },
Primitive{ values: Buffer },
Offsets { offsets: Buffer, values: Buffer },
Dictionary { keys: Buffer, values: ArrayData },
List { offsets: Buffer, elements: ArrayData },
Struct { children: Vec<ArrayData> },
Union { offsets: Option<Buffer>, types: Buffer, children: Vec<ArrayData> },
}
```
```
pub struct ArrayData {
/// The data type for this array data
data_type: DataType,
/// The number of elements in this array data
len: usize,
/// The number of null elements in this array data
null_count: usize,
/// The offset into this array data, in number of items
offset: usize,
/// The null bitmap. A `None` value for this indicates all values are
non-null in
/// this array.
null_bitmap: Option<Bitmap>,
/// The array data layout
layout: ArrayDataLayout
}
```
We could then progressively deprecate the methods that explicitly refer to
buffers by index, etc...
**Describe alternatives you've considered**
We could not do this
**Additional context**
This could be seen as an evolution of @HaoYang670 's proposal in
https://github.com/apache/arrow-rs/issues/1640
It also relates to @jhorstmann 's proposal on
https://github.com/apache/arrow-rs/pull/1499#issuecomment-1096878229
It could also be seen as an interpretation of the arrow2 physical vs logical
type separation.
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