tustvold opened a new issue, #5426:
URL: https://github.com/apache/arrow-rs/issues/5426
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Currently `build_compare` can be used to construct a `DynComparator` that
can be used to establish an ordering between two different array indices. It is
up to the caller to inspect the null masks, and apply ordering as appropriate.
This works well for arrays non-nested arrays, but runs into issues when arrays
have children.
The introduction of logical nullability worked around this limitation for
DictionaryArray and RunArray, but it is unclear how best to handle types such
as StructArray and ListArray.
Part of the challenge is there isn't one way users may wish to handle nulls:
* When sorting nulls are ordered based on the `SortOptions::nulls_first`
parameter
* When used in a comparison operator, the behaviour of nulls depends on the
operator
Also correctly using `DynComparator` requires understanding the distinction
between logical and physical nullability, see #4691, which is confusing (#4840)
**Describe the solution you'd like**
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In order to support arbitrarily nested ListArray in comparison kernels, we
need a mechanism similar to DynComparator but which is able to handle nulls.
I would like something similar to the below
```
pub enum Compare {
LeftNull,
RightNull,
BothNull,
Less,
Equal,
Greater,
}
impl Compare {
pub fn ordering(&self, nulls_first: bool) -> Ordering {
...
}
#[inline]
pub fn is_null(&self) -> bool {
matches!(self, Self::LeftNull | Self::RightNull | Self::BothNull)
}
}
pub type Comparator = Box<dyn Fn(usize, usize) -> Compare + Send + Sync>;
pub fn build_comparator(left: &dyn Array, right: &dyn Array) ->
Result<Comparator> {
...
}
```
We could then deprecate and eventually remove `build_compare` and
`DynComparator`.
**Describe alternatives you've considered**
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