zeroshade commented on code in PR #47456: URL: https://github.com/apache/arrow/pull/47456#discussion_r2331124199
########## docs/source/format/CanonicalExtensions.rst: ########## @@ -417,7 +421,133 @@ better zero-copy compatibility with various systems that also store booleans usi Metadata is an empty string. -========================= +.. _parquet_variant_extension: + +Parquet Variant +=============== + +Variant represents a value that may be one of: + +* Primitive: a type and corresponding value (e.g. ``INT``, ``STRING``) + +* Array: An ordered list of Variant values + +* Object: An unordered collection of string/Variant pairs (i.e. key/value pairs). An object may not contain duplicate keys + +Particularly, this provides a way to represent semi-structured data which is stored as a +`Parquet Variant <https://github.com/apache/parquet-format/blob/master/VariantEncoding.md>`__ value within Arrow columns in +a lossless fashion. This also provides the ability to represent `shredded <https://github.com/apache/parquet-format/blob/master/VariantShredding.md>`__ +variant values. This will make it possible for systems to pass Variant data around without having to upgrade their Arrow version +or otherwise require special handling unless they want to directly interact with the encoded variant data. See the previous links +to the Parquet format specification for details on what the actual binary values should look like. + +* Extension name: ``arrow.parquet.variant``. + +* The storage type of this extension is a ``Struct`` that obeys the following rules: + + * A *non-nullable* field named ``metadata`` which is of type ``Binary``, ``LargeBinary``, or ``BinaryView``. + + * At least one (or both) of the following: + + * A field named ``value`` which is of type ``Binary``, ``LargeBinary``, or ``BinaryView``. + (unshredded variants consist of just the ``metadata`` and ``value`` fields only) + + * A field named ``typed_value`` which can be a :ref:`variant_primitive_type_mapping` or a ``List``, ``LargeList``, ``ListView`` or ``Struct`` + + * If the ``typed_value`` field is a ``List``, ``LargeList`` or ``ListView`` its elements **must** be *non-nullable* and **must** + be a ``Struct`` consisting of at least one (or both) of the following: + + * A field named ``value`` which is of type ``Binary``, ``LargeBinary``, or ``BinaryView``. + + * A field named ``typed_value`` which follows the rules outlined above (this allows for arbitrarily nested data). + + * If the ``typed_value`` field is a ``Struct``, then its fields **must** be *non-nullable*, representing the fields being shredded + from the objects, and **must** be a ``Struct`` consisting of at least one (or both) of the following: + + * A field named ``value`` which is of type ``Binary``, ``LargeBinary``, or ``BinaryView``. + + * A field named ``typed_value`` which follows the rules outlined above (this allows for arbitrarily nested data). + +* Extension type parameters: + + This type does not have any parameters. + +* Description of the serialization: + + Extension metadata is an empty string. + +.. note:: + + It is also *permissible* for the ``metadata`` field to be dictionary-encoded with a preferred (*but not required*) index type of ``int8``, + or run-end-encoded with a preferred (*but not required*) runs type of ``int8``. + +.. note:: + + The fields may be in any order, and thus must be accessed by **name** not by *position*. The field names are case sensitive. + +.. _variant_primitive_type_mapping: + +Primitive Type Mappings +----------------------- + ++----------------------+------------------------+ +| Arrow Primitive Type | Variant Primitive Type | ++======================+========================+ +| Null | Null | ++----------------------+------------------------+ +| Boolean (true/false) | Boolean | Review Comment: good point, my mistake there. I'll swap these -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: github-unsubscr...@arrow.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
