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Multi-file datasets

Source: R/dataset.R, R/dataset-factory.R
Dataset.Rd

Arrow Datasets allow you to query against data that has been split across multiple files. This sharding of data may indicate partitioning, which can accelerate queries that only touch some partitions (files).

A Dataset contains one or more Fragments, such as files, of potentially differing type and partitioning.

For Dataset$create(), see open_dataset(), which is an alias for it.

DatasetFactory is used to provide finer control over the creation of Datasets.

Factory

DatasetFactory is used to create a Dataset, inspect the Schema of the fragments contained in it, and declare a partitioning. FileSystemDatasetFactory is a subclass of DatasetFactory for discovering files in the local file system, the only currently supported file system.

For the DatasetFactory$create() factory method, see dataset_factory(), an alias for it. A DatasetFactory has:

  • $Inspect(unify_schemas): If unify_schemas is TRUE, all fragments will be scanned and a unified Schema will be created from them; if FALSE (default), only the first fragment will be inspected for its schema. Use this fast path when you know and trust that all fragments have an identical schema.

  • $Finish(schema, unify_schemas): Returns a Dataset. If schema is provided, it will be used for the Dataset; if omitted, a Schema will be created from inspecting the fragments (files) in the dataset, following unify_schemas as described above.

FileSystemDatasetFactory$create() is a lower-level factory method and takes the following arguments:

  • filesystem: A FileSystem

  • selector: Either a FileSelector or NULL

  • paths: Either a character vector of file paths or NULL

  • format: A FileFormat

  • partitioning: Either Partitioning, PartitioningFactory, or NULL

Methods

A Dataset has the following methods:

  • $NewScan(): Returns a ScannerBuilder for building a query

  • $WithSchema(): Returns a new Dataset with the specified schema. This method currently supports only adding, removing, or reordering fields in the schema: you cannot alter or cast the field types.

  • $schema: Active binding that returns the Schema of the Dataset; you may also replace the dataset's schema by using ds$schema <- new_schema.

FileSystemDataset has the following methods:

  • $files: Active binding, returns the files of the FileSystemDataset

  • $format: Active binding, returns the FileFormat of the FileSystemDataset

UnionDataset has the following methods:

  • $children: Active binding, returns all child Datasets.

See also

open_dataset() for a simple interface to creating a Dataset