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From "Calder, Matthew" <mcal...@xbktrading.com>
Subject RE: Converting clickhouse column to arrow array
Date Wed, 29 Jan 2020 15:55:22 GMT
I managed to get conversion from CH to arrow using a CHToArrowType<> inter-type traits
concept. However, I am still trying to crack the use of:

 arrow::VisitArrayInline

and 

arrow::ArrayDataVisitor


I have a struct:

Struct AnArrayUser
{
     template <typename T> arrow::Status Visit(const T &a)
     {
           // How to invoke ArrayDataVisitor?
     }

     void Use(const arrow::Array &a) {arrow::VisitArrayInline(a, this);}


     arrow::Status VisitNull() {return arrow::Status::OK();}
     template <class T> arrow::Status VisitValue(T val) {return arrow::Status::OK();}
};

Which appears to have it's "Use" method called appropriately. But inside of the Visit method
I have so far been unable to find the incantation to make a call through the ArrayDataVisitor.
I've tried several variations of:

arrow::ArrayDataVisitor<typename T::TypeClass>::Visit(*(array.data()), this);

at the // How to .. line above but can't seem to get it to work. I'm sure I just have some
fundamental misunderstanding of how this is supposed to work. Can someone give me some guidance?

Matt

   

-----Original Message-----
From: Wes McKinney <wesmckinn@gmail.com> 
Sent: Wednesday, January 22, 2020 12:03 PM
To: user@arrow.apache.org
Subject: Re: Converting clickhouse column to arrow array

If you search for "VisitTypeInline" or "VisitArrayInline" in the C++ codebase you can find
numerous examples of where this is used

On Wed, Jan 22, 2020 at 10:58 AM Thomas Buhrmann <thomas.buehrmann@gmail.com> wrote:
>
> Hi,
> I was looking for something similar, but didn't find a good example in the docs or the
source code showing how to use the visitor pattern. It would be great, e.g., to have an example
similar to the "Row to columnar conversion", showing a templated way to read arrow columns
into C++ vectors using the visitor pattern, and without implementing a separate reader function
for each arrow type. Would that be possible?
>
> Many thanks,
> Thomas
>
> On Wed, 22 Jan 2020 at 17:13, Wes McKinney <wesmckinn@gmail.com> wrote:
>>
>> hi Matt,
>>
>> I recommend you use the visitor pattern combined with the 
>> arrow::TypeTraits that we provide
>>
>> https://clicktime.symantec.com/38JEFUTGByJzrxbCs1aM2Mn7Vc?u=https%3A%
>> 2F%2Fgithub.com%2Fapache%2Farrow%2Fblob%2Fmaster%2Fcpp%2Fsrc%2Farrow%
>> 2Ftype_traits.h
>>
>> You'll need to provide a compile-time mapping from Clickhouse types 
>> to Arrow types, but then you can statically access the correct 
>> builder type at compile time
>>
>> using ArrowType = typename CHToArrowType<CHType>::ArrowType; using 
>> BuilderType = typename TypeTraits<ArrowType>::BuilderType;
>>
>> ...
>>
>> or similar. In cases where the exported Clickhouse data does not have 
>> an associated AppendValues method in Arrow you may have to write a 
>> special case (please open JIRA issues if you think there should be 
>> more AppendValues methods)
>>
>> Thanks
>>
>> On Wed, Jan 22, 2020 at 7:44 AM Calder, Matthew <mcalder@xbktrading.com> wrote:
>> >
>> > Hi,
>> >
>> >
>> >
>> > I am interfacing arrow to a Clickhouse database using their c++ client. Both
arrow and CH have generic array-like classes with the element data type internalized. Ideally,
I would like to be able to write something like:
>> >
>> >
>> >
>> > arrow::Array a = SomeConversionInvocation(clickhouse::Column c);
>> >
>> >
>> >
>> > Where the array and column have the same element type (int, double, string,
…) but the code is generic to the specific type.
>> >
>> >
>> >
>> > I can do this by explicitly handling specific types through template specialization
but I thought that since arrow already has pretty generic type handling through its templates,
and clickhouse also has similar capability there ought to be a more seamless way to do the
conversion. Zero copy would probably be a lot to ask, but something short of template specializations
for every type is what I am aiming for.
>> >
>> >
>> >
>> > I currently do explicit type specialization. For example I have functions like:
>> >
>> >
>> >
>> > inline std::shared_ptr<arrow::Array> makeArray(const 
>> > std::vector<double> &v)
>> >
>> > {
>> >
>> >     arrow::DoubleBuilder builder;
>> >
>> >     builder.AppendValues(v);
>> >
>> >     std::shared_ptr<arrow::Array> array;
>> >
>> >     builder.Finish(&array);
>> >
>> >     return array;
>> >
>> > }
>> >
>> >
>> >
>> > inline std::shared_ptr<arrow::Array> makeArray(const 
>> > std::vector<int> &v)
>> >
>> > {
>> >
>> >     arrow::Int32Builder builder;
>> >
>> >     builder.AppendValues(v);
>> >
>> >     std::shared_ptr<arrow::Array> array;
>> >
>> >     builder.Finish(&array);
>> >
>> >     return array;
>> >
>> > }
>> >
>> >
>> >
>> > Which I suspect is unnecessarily explicit. Is there a more generic way of handling
the variety of underlying array element data types when constructing arrow::Array objects?
And can someone point me to examples that interface arrow to another similarly generically
typed library (doesn’t have to be clickhouse). Thanks for any guidance.
>> >
>> >
>> >
>> > Matt
>> >
>> >
>> >
>> >
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