Can't cast from timestamp array to string array

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I am trying to work with timestamps -- specifically converting data from csv, etc to timestamps

Describe the solution you'd like
I would like the cast kernel to support convertingTimestampNanosecondArray (and the other date / time / timestamp related types) to StringArray

I would like the following Rust code to compile and produce a String array with value "1970-01-01 00:00:00.000000100":
Reproducer:

    let array = TimestampNanosecondArray::from(vec![100]);
    let array: ArrayRef = Arc::new(array);
    // array[0]: "1970-01-01 00:00:00.000000100"
    println!("array[0]: {:?}", array_value_to_string(&array, 0).unwrap());

    // cast to string array results in an error:
    // thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: CastError("Casting from Timestamp(Nanosecond, None) to Utf8 not supported")', src/main.rs:26:54
    let string_array = cast(&array, &DataType::Utf8).unwrap();

Additional context

The code exists in arrow to convert strings to timestamps (string_to_timestamp_nanos) and timestamps to strings, but the code is not connected to the cast kernel

Thus, it is not easy to convert an array of timestamps to an array of strings

[sum12]

Hello, Thanks for the awesome description.
I spent sometime reading through the code base to implement this cast.

• one way to would be have a helper cast_timestamp_to_string helper which iterates over each of elements and calls array_value_to_string and collect result and into it into GenericStringArray

• or one could replicate what as_datetime does in the cast.rs but limit it to Timestamp(_,_) type

• or one could just cast the array into a PrimitiveArray and call value_as_time and push into a Vec<String> and into a GenericStringArray (This seem to be the simplest approch but I am not sure how to bound it to ArrowTimestampType only since value_as_datetime expect to satisfy ArrowNumericType also)

I tried 2 and 3 but still wasnt sure.

[alamb]

Thanks @sum12 ! think the trait you want is ArrowTemporalType https://docs.rs/arrow/5.1.0/arrow/datatypes/trait.ArrowTemporalType.html

So then you could write a function like:

fn temporal_array_to_string<T>(array: &PrimitiveArray<T>) -> Result<ArrayRef>
where
    T: ArrowTemporalType,
{
  //  ... call array.value_as_datetime  here as appropriate
}

And then to call it you might do something like:

let timestamp_array = array.as_any().downcast_ref::<TimestampNanosecondArray>().unwrap();
temporal_array_to_string(timestamp_array)

[alamb]

I also updated the description to be consistent (the example code and description said the opposite)