kou commented on code in PR #42133:
URL: https://github.com/apache/arrow/pull/42133#discussion_r1641570658
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cpp/src/arrow/array/array_base.h:
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@@ -232,6 +232,14 @@ class ARROW_EXPORT Array {
/// \return DeviceAllocationType
DeviceAllocationType device_type() const { return data_->device_type(); }
+ /// \brief Return the statistics of this Array
+ ///
+ /// This just delegates to calling statistics on the underlying ArrayData
+ /// object which backs this Array.
+ ///
+ /// \return const ArrayStatistics&
+ const ArrayStatistics& statistics() const { return data_->statistics; }
Review Comment:
> Should statistics be stored in memory together with every `ArrayData`
instance?
If it's not desired, we can avoid it by using
`std::shared_ptr<ArrayStatistics>` or something.
> Another problem with this is that statistics are derived data and
`ArraData` is mutable when manipulated directly, so any mutation of `ArrayData`
will have to consider the consequences to the derived statistics.
>
> Lazily-computed `null_count_` is a source of bugs and complexity for this
reason. IMO statistics should be (1) computed or (2) carried from a file
readers (like Parquet's) as something on the side.
How about attaching the statistics read by a file reader to `Array` (not
`ArrayData`) directly?
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