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From Antoine Pitrou <anto...@python.org>
Subject Re: [Python] Efficient numpy.recarray to pyarrow.StructArray conversion
Date Sun, 21 Mar 2021 13:51:18 GMT
On Sun, 21 Mar 2021 12:33:09 +0000
Hagai Har-Gil <hagaihargil@protonmail.com> wrote:
> After some more digging I did arrive at something which seems more efficient than what
I had:
> 
> struct_schema = pa.struct([('field0', pa.int32()), ('field1', pa.int8())])
> nparray = x = np.array([(0, 10), (1, 20)], dtype=[('field0', '<i4'), ('field1', '<i1')])
> struct_array = pa.array(nparray, type=struct_schema)
> 
> This looks easy, although I'm not sure how much copying is done down below.

The data is definitely copied under the hood, since this is
converting from an "array of structs" layout (the Numpy array) to a
"struct of arrays" layout (the Arrow array).

This is a conceptual constraint.  I don't think it is possible to
create a Numpy struct array that would use separate data areas for the
struct fields.

Regards

Antoine.



> 
> I now have an issue with the Rust implementation since I'm not sure how do I access or
iterate over the rows of the resulting StructArray, which was trivial in Python.
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Sunday, March 21, 2021 2:22 PM, Hagai Har-Gil <hagaihargil@protonmail.com> wrote:
> 
> > After some more digging I did arrive at something which seems more efficient than
what I had:
> >
> > struct_schema = pa.struct([('field0', pa.int32()), ('field1', pa.int8())])
> > nparray = x = np.array([(0, 10), (1, 20)], dtype=[('field0', '<i4'), ('field1',
'<i1')])
> > struct_array = pa.array(nparray, type=struct_schema)
> >
> > This looks easy, although I'm not sure how much copying is done down below.
> >
> > I now have an issue with the Rust implementation since I'm not sure how do I access
or iterate over the rows of the resulting StructArray.
> >
> > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> > On Sunday, March 21, 2021 10:52 AM, Hagai Har-Gil <hagaihargil@protonmail.com>
wrote:
> >  
> >> Hi,
> >>
> >> I'm trying to efficiently convert incoming numpy.recarray's to pyarrow.StructArray
and I'm unsure how to do so with the least amount of copying.
> >>
> >> My use case involves real time data processing of numpy.recarrays in Rust. I'm
happily using the IPC protocol to transfer data to Rust's arrow implementation which will
do the heavy lifting. I'll need to iterate on the recarray-turned-StructArray line-by-line,
each time yielding all fields of a specific row, so the StructArray format is quite fitting.
However, doing the actual conversion in an efficient manner seems harder than expected. The
fields (=individual arrays) of a numpy.recarray aren't stored in a contiguous manner, so any
numpy.recarray -> pyarrow.Array conversion first has to copy the data to standard pyarrow.Array
buffers, and then re-construct the StructArray structure by interleaving the arrays. I was
unable to find in the docs or in previous discussions here a better approach for this type
of pre-processing step.
> >>
> >> Since I'm using IPC I'll eventually need to have the pyarrow.StructArray wrapped
in a pyarrow.RecordBatch if that makes any difference.
> >>
> >> Thanks in advance  




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