Hi,
I implement the first version of arrow::ArrayStatistics:
https://github.com/apache/arrow/pull/42133
The PR description and this e-mail explain discussion
points. Could you share your comments?
Implementation:
arrow::ArrayStatistics:
----
/// \brief Statistics for an Array
///
/// Apache Arrow format doesn't have statistics but data source such
/// as Apache Parquet may have statistics. Statistics associate with
/// data source can be read unified API via this class.
struct ARROW_EXPORT ArrayStatistics {
public:
// DISCUSS:
// * How do we support binary/text data?
// * Should we use arrow::Scalar instead?
using ElementBufferType = std::variant<bool, int8_t, uint8_t, int16_t,
uint16_t,
int32_t, uint32_t, int64_t, uint64_t>;
ArrayStatistics() = default;
~ArrayStatistics() = default;
/// \brief The number of null values, may not be set
std::optional<int64_t> null_count = std::nullopt;
/// \brief The number of distinct values, may not be set
std::optional<int64_t> distinct_count = std::nullopt;
/// \brief The current minimum value buffer, may not be set
std::optional<ElementBufferType> min_buffer = std::nullopt;
/// \brief The current maximum value buffer, may not be set
std::optional<ElementBufferType> max_buffer = std::nullopt;
/// \brief Check two Statistics for equality
bool Equals(const ArrayStatistics& other) const {
return null_count == other.null_count && distinct_count ==
other.distinct_count &&
min_buffer == other.min_buffer && max_buffer == other.max_buffer;
}
};
/// \brief A typed implementation of ArrayStatistics for convenience
template <typename TypeClass>
class TypedArrayStatistics : public ArrayStatistics {
public:
using ElementType = typename TypeClass::c_type;
/// \brief The current minimum value, may not be set
std::optional<ElementType> min() const {
if (min_buffer && std::holds_alternative<ElementType>(*min_buffer)) {
return std::get<ElementType>(*min_buffer);
} else {
return std::nullopt;
}
}
/// \brief The current maximum value, may not be set
std::optional<ElementType> max() const {
if (max_buffer && std::holds_alternative<ElementType>(*max_buffer)) {
return std::get<ElementType>(*max_buffer);
} else {
return std::nullopt;
}
}
};
----
arrow::ArrayData has arrow::ArrayStatistics and
arrow::Array::statistics() returns it:
*
https://github.com/apache/arrow/pull/42133/files#diff-c5d8f67626f30c66a79d1f677ee81264a9b044280e942cfa846e07306b25365bR400
*
https://github.com/apache/arrow/pull/42133/files#diff-5557ff71f88c165e161f89fbb390e1c1fef70d067a892475aa77471b13e1e322R241
Points:
arrow::ArrayData not arrow::Array has arrow::ArrayStatistics
to keep backward compatibility and
extensibility. arrow::Array is immutable. So we need to set
all values in constructor. The current arrow::*Array
constructors have many arguments. Adding one more argument
for arrow::ArrayStatistics breaks ABI and reduces usability.
If arrow::ArrayData has arrow::ArrayStatistics, we can
prepare arrow::ArrayStatistics out of constructor with
arrow::ArrayData. Because arrow::ArrayData is mutable. So we
can change arrow::ArrayData not in constructor.
Limitations:
This supports only the following types for now:
* BooleanType
* Int*Type
* UInt*Type
BinaryType and StringType family support will be difficult
than them because it requires variable size storage.
Demo:
This includes a demo how to associate statistics read from
Apache Parquet data to
arrow::BooleanArray/arrow::Int*Array/arrow::UInt*Array.
https://github.com/apache/arrow/pull/42133/files#diff-1adfd998f8b1d8aef97c028e7f8d49fea94dbf42481d640be34e488d51581c0e
Discussions:
* How do we support min/max of binary/text data?
* Parquet has FLBAStatistics (FLBA = Fixed Length Byte Array).
* Can we use std::string_view for it?
* If so, where do we store the original value?
* Can we use std::string?
* It needs a copy and copying large data will not be desired.
* Should we use arrow::Scalar not std::variant<bool, int8_t, ...>?
* If we use arrow::Scalar, we can support complex value
such as list and struct. But arrow::Scalar may be
heavy.
* Should this be opt-in?
Thanks,
--
kou
In <20240609.161934.1193014034909936181....@clear-code.com>
"Re: [DISCUSS][C++] How about adding arrow::ArrayStatistics?" on Sun, 09 Jun
2024 16:19:34 +0900 (JST),
Sutou Kouhei <k...@clear-code.com> wrote:
> Hi,
>
> OK. I'll propose arrow::ArrayStatistics API that can be used
> as a starting point.
>
>
> Thanks,
> --
> kou
>
> In <cak7z5t8qsk0qnnwez4tbpj9x1p2oqy-t6karh-jh4zbjhe9...@mail.gmail.com>
> "Re: [DISCUSS][C++] How about adding arrow::ArrayStatistics?" on Wed, 5 Jun
> 2024 22:55:25 -0700,
> Micah Kornfield <emkornfi...@gmail.com> wrote:
>
>> Generally I think this is a good idea that has been proposed before but I
>> don't think we could ever make progress on design.
>>
>> On Sun, Jun 2, 2024 at 7:17 PM Sutou Kouhei <k...@clear-code.com> wrote:
>>
>>> Hi,
>>>
>>> Related GitHub issue:
>>> https://github.com/apache/arrow/issues/41909
>>>
>>> How about adding arrow::ArrayStatistics?
>>>
>>> Motivation:
>>>
>>> An Apache Arrow format data doesn't have statistics. (We can
>>> add statistics as metadata but there isn't any standard way
>>> for it.)
>>>
>>> But a source of an Apache Arrow format data such as Apache
>>> Parquet format data may have statistics. We can get the
>>> source statistics via source reader such as
>>> parquet::ColumnChunkMetaData::statistics() but can't get
>>> them from read Apache Arrow format data. If we want to use
>>> the source statistics, we need to keep the source reader.
>>>
>>> Proposal:
>>>
>>> How about adding arrow::ArrayStatistics or something and
>>> attaching source statistics to read arrow::Array? If source
>>> statistics are attached to read arrow::Array, we don't need
>>> to keep a source reader to get source statistics.
>>>
>>> What do you think about this idea?
>>>
>>>
>>> NOTE: I haven't thought about the arrow::ArrayStatistics
>>> details yet. We'll be able to use parquet::Statistics and
>>> its family as a reference.
>>> https://github.com/apache/arrow/blob/main/cpp/src/parquet/statistics.h
>>>
>>>
>>> Thanks,
>>> --
>>> kou
>>>
