The central type in Apache Arrow are arrays, which are a known-length sequence of values
all having the same type. This crate provides concrete implementations of each type, as
well as an [Array
] trait that can be used for type-erasure.
Building an Array
Most [Array
] implementations can be constructed directly from iterators or [Vec
]
# use ;
# use Int32Type;
#
from;
from;
from_iter;
from_iter;
from;
from;
from_iter;
from_iter_values;
;
Additionally ArrayBuilder
implementations can be
used to construct arrays with a push-based interface
# use Int16Array;
#
// Create a new builder with a capacity of 100
let mut builder = builder;
// Append a single primitive value
builder.append_value;
// Append a null value
builder.append_null;
// Append a slice of primitive values
builder.append_slice;
// Build the array
let array = builder.finish;
assert_eq!;
assert_eq!;
assert_eq!
Low-level API
Internally, arrays consist of one or more shared memory regions backed by a Buffer
,
the number and meaning of which depend on the array’s data type, as documented in
the Arrow specification.
For example, the type [Int16Array
] represents an array of 16-bit integers and consists of:
- An optional
NullBuffer
identifying any null values - A contiguous
ScalarBuffer<i16>
of values
Similarly, the type [StringArray
] represents an array of UTF-8 strings and consists of:
- An optional
NullBuffer
identifying any null values - An offsets
OffsetBuffer<i32>
identifying valid UTF-8 sequences within the values buffer - A values
Buffer
of UTF-8 encoded string data
Array constructors such as [PrimitiveArray::try_new
] provide the ability to cheaply
construct an array from these parts, with functions such as [PrimitiveArray::into_parts
]
providing the reverse operation.
# use ;
# use OffsetBuffer;
#
// Create a Int32Array from Vec without copying
let array = new;
assert_eq!;
assert_eq!;
// Create a StringArray from parts
let offsets = new;
let array = new;
let values: = array.iter.map.collect;
assert_eq!;
As Buffer
, and its derivatives, can be created from [Vec
] without copying, this provides
an efficient way to not only interoperate with other Rust code, but also implement kernels
optimised for the arrow data layout - e.g. by handling buffers instead of values.
Zero-Copy Slicing
Given an [Array
] of arbitrary length, it is possible to create an owned slice of this
data. Internally this just increments some ref-counts, and so is incredibly cheap
# use Int32Array;
let array = from_iter;
// Slice with offset 1 and length 2
let sliced = array.slice;
assert_eq!;
Downcasting an Array
Arrays are often passed around as a dynamically typed &dyn Array
or [ArrayRef
].
For example, RecordBatch
stores columns as [ArrayRef
].
Whilst these arrays can be passed directly to the compute
, csv
, json
, etc... APIs,
it is often the case that you wish to interact with the concrete arrays directly.
This requires downcasting to the concrete type of the array:
# use ;
// Safely downcast an `Array` to an `Int32Array` and compute the sum
// using native i32 values
// Safely downcasts the array to a `Float32Array` and returns a &[f32] view of the data
// Note: the values for positions corresponding to nulls will be arbitrary (but still valid f32)
The [cast::AsArray
] extension trait can make this more ergonomic
# use Array;
# use ;
# use Float32Type;
Alternatives to ChunkedArray Support
The Rust implementation does not provide the ChunkedArray abstraction implemented by the Python and C++ Arrow implementations. The recommended alternative is to use one of the following:
Vec<ArrayRef>
a simple, eager version of aChunkedArray
impl Iterator<Item=ArrayRef>
a lazy version of aChunkedArray
impl Stream<Item=ArrayRef>
a lazy async version of aChunkedArray
Similar patterns can be applied at the RecordBatch
level. For example, DataFusion makes
extensive use of RecordBatchStream.
This approach integrates well into the Rust ecosystem, simplifies the implementation and encourages the use of performant lazy and async patterns.
use Arc;
use ;
use AsArray;
use Float32Type;
use DataType;
let batches = ;
let labels: = batches
.iter
.flat_map
.map
.collect;
let values: = batches
.iter
.flat_map
.copied
.collect;
assert_eq!;
assert_eq!;