Struct FileDecoder

pub struct FileDecoder { /* private fields */ }

A low-level, push-based interface for reading an IPC file

For a higher-level interface see FileReader

For an example of using this API with mmap see the zero_copy_ipc example.

// Write an IPC file

let batch = RecordBatch::try_from_iter([
    ("a", Arc::new(Int32Array::from(vec![1, 2, 3])) as _),
    ("b", Arc::new(Int32Array::from(vec![1, 2, 3])) as _),
    ("c", Arc::new(DictionaryArray::<Int32Type>::from_iter(["hello", "hello", "world"])) as _),
]).unwrap();

let schema = batch.schema();

let mut out = Vec::with_capacity(1024);
let mut writer = FileWriter::try_new(&mut out, schema.as_ref()).unwrap();
writer.write(&batch).unwrap();
writer.finish().unwrap();

drop(writer);

// Read IPC file

let buffer = Buffer::from_vec(out);
let trailer_start = buffer.len() - 10;
let footer_len = read_footer_length(buffer[trailer_start..].try_into().unwrap()).unwrap();
let footer = root_as_footer(&buffer[trailer_start - footer_len..trailer_start]).unwrap();

let back = fb_to_schema(footer.schema().unwrap());
assert_eq!(&back, schema.as_ref());

let mut decoder = FileDecoder::new(schema, footer.version());

// Read dictionaries
for block in footer.dictionaries().iter().flatten() {
    let block_len = block.bodyLength() as usize + block.metaDataLength() as usize;
    let data = buffer.slice_with_length(block.offset() as _, block_len);
    decoder.read_dictionary(&block, &data).unwrap();
}

// Read record batch
let batches = footer.recordBatches().unwrap();
assert_eq!(batches.len(), 1); // Only wrote a single batch

let block = batches.get(0);
let block_len = block.bodyLength() as usize + block.metaDataLength() as usize;
let data = buffer.slice_with_length(block.offset() as _, block_len);
let back = decoder.read_record_batch(block, &data).unwrap().unwrap();

assert_eq!(batch, back);

Implementations

impl FileDecoder

pub fn new(schema: Arc<Schema>, version: MetadataVersion) -> FileDecoder

Create a new FileDecoder with the given schema and version

pub fn with_projection(self, projection: Vec<usize>) -> FileDecoder

Specify a projection

pub fn with_require_alignment(self, require_alignment: bool) -> FileDecoder

Specifies if the array data in input buffers is required to be properly aligned.

If require_alignment is true, this decoder will return an error if any array data in the input buf is not properly aligned. Under the hood it will use arrow_data::ArrayDataBuilder::build to construct arrow_data::ArrayData.

If require_alignment is false (the default), this decoder will automatically allocate a new aligned buffer and copy over the data if any array data in the input buf is not properly aligned. (Properly aligned array data will remain zero-copy.) Under the hood it will use arrow_data::ArrayDataBuilder::build_aligned to construct arrow_data::ArrayData.

pub unsafe fn with_skip_validation(self, skip_validation: bool) -> FileDecoder

Specifies if validation should be skipped when reading data (defaults to false)

Safety

This flag must only be set to true when you trust the input data and are sure the data you are reading is a valid Arrow IPC file, otherwise undefined behavior may result.

For example, some programs may wish to trust reading IPC files written by the same process that created the files.

pub fn read_dictionary( &mut self, block: &Block, buf: &Buffer, ) -> Result<(), ArrowError>

Read the dictionary with the given block and data buffer

pub fn read_record_batch( &self, block: &Block, buf: &Buffer, ) -> Result<Option<RecordBatch>, ArrowError>

Read the RecordBatch with the given block and data buffer

Trait Implementations

impl Debug for FileDecoder

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.