kou commented on code in PR #47456: URL: https://github.com/apache/arrow/pull/47456#discussion_r2317613597
########## docs/source/format/CanonicalExtensions.rst: ########## @@ -417,7 +421,591 @@ better zero-copy compatibility with various systems that also store booleans usi Metadata is an empty string. -========================= +.. _variant_extension: Review Comment: ```suggestion .. _parquet_variant_extension: ``` ########## docs/source/format/CanonicalExtensions.rst: ########## @@ -417,7 +421,591 @@ better zero-copy compatibility with various systems that also store booleans usi Metadata is an empty string. -========================= +.. _variant_extension: + +Parquet Variant +=============== + +Variant represents a value that may be one of: + +* Primitive: a type and corresponding value (e.g. INT, STRING) Review Comment: ```suggestion * Primitive: a type and corresponding value (e.g. ``INT``, ``STRING``) ``` ########## docs/source/format/CanonicalExtensions.rst: ########## @@ -417,7 +421,591 @@ better zero-copy compatibility with various systems that also store booleans usi Metadata is an empty string. -========================= +.. _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: ``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 any :term:`primitive type` or a ``List``, ``LargeList``, ``ListView`` or ``Struct`` + + * If the ``typed_value`` field is a *nested* type, 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). +* 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``. + +.. note:: + + The fields may be in any order, and thus must be accessed by **name** not by *position*. The field names are case sensitive. + +Examples +-------- + +Unshredded +'''''''''' + +The simplest case, an unshredded variant always consists of **exactly** two fields: ``metadata`` and ``value``. Any of +the following storage types are valid (not an exhaustive list): + +* ``struct<metadata: binary non-nullable, value: binary nullable>`` +* ``struct<value: binary nullable, metadata: binary non-nullable>`` +* ``struct<metadata: dictionary<int8, binary> non-nullable, value: binary_view nullable>`` + +Simple Shredding +'''''''''''''''' + +Suppose we have a Variant field named *measurement* and we want to shred the ``int64`` values into a separate column for efficiency. +In Parquet, this could be represented as:: + + required group measurement (VARIANT) { + required binary metadata; + optional binary value; + optional int64 typed_value; + } + +Thus the corresponding storage type for the ``parquet.variant`` Arrow extension type would be:: + + struct< + metadata: binary required, + value: binary optional, + typed_value: int64 optional + > + +If we suppose a series of measurements consisting of:: + + 34, null, "n/a", 100 + +The data should be stored/represented in Arrow as:: + + * Length: 4, Null count: 1 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00001011 | 0 (padding) | + + * Children arrays: + * field-0 array (`VarBinary`) + * Length: 4, Null count: 0 + * Offsets buffer: + + | Bytes 0-19 | Bytes 20-63 | + |------------------|--------------------------| + | 0, 2, 4, 6, 8 | unspecified (padding) | + + * Value buffer: (01 00 -> indicates version 1 empty metadata) + + | Bytes 0-7 | Bytes 8-63 | + |-------------------------|--------------------------| + | 01 00 01 00 01 00 01 00 | unspecified (padding) | + + * field-1 array (`VarBinary`) + * Length: 4, Null count: 2 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00000110 | 0 (padding) | + + * Offsets buffer: + + | Bytes 0-19 | Bytes 20-63 | + |------------------|--------------------------| + | 0, 0, 1, 5, 5 | unspecified (padding) | + + * Value buffer: (`00` -> literal null, `0x13 0x6E 0x2F 0x61` -> variant encoding literal string "n/a") + + | Bytes 0-4 | Bytes 5-63 | + |------------------------|--------------------------| + | 00 0x13 0x6E 0x2F 0x61 | unspecified (padding) | + + * field-2 array (int64 array) + * Length: 4, Null count: 2 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00001001 | 0 (padding) | + + * Value buffer: + + | Bytes 0-31 | Bytes 32-63 | + |---------------------|--------------------------| + | 34, 00, 00, 100 | unspecified (padding) | + +.. note:: + + Notice that there is a variant ``literal null`` in the ``value`` array, this is due to the + `shredding specification <https://github.com/apache/parquet-format/blob/master/VariantShredding.md#value-shredding>`__ + so that a consumer can tell the difference between a *missing* field and a **null** field. A null + element must be encoded as a Variant null: *basic type* ``0`` (primitive) and *physical type* ``0`` (null). + +Shredding an Array +'''''''''''''''''' + +For our next example, we will represent a shredded array of strings. Let's consider a column that looks like: :: + + ["comedy", "drama"], ["horror", null], ["comedy", "drama", "romance"], null + +Representing this shredded variant in Parquet could look like:: + + optional group tags (VARIANT) { + required binary metadata; + optional binary value; + optional group typed_value (LIST) { # optional to allow null lists + repeated group list { + required group element { # shredded element + optional binary value; + optional binary typed_value (STRING); + } + } + } + } + +The array structure for Variant encoding does not allow missing elements, so all elements of the array must +be *non-nullable*. As such, either **typed_value** or **value** (*but not both!*) must be *non-null*. + +The storage type to represent this in Arrow as a Variant extension type would be:: + + struct< + metadata: binary non-nullable, + value: binary nullable, + typed_value: list<element: struct< + value: binary nullable, + typed_value: string nullable + > required> nullable + > + +.. note:: + + As usual, **Binary** could also be **LargeBinary** or **BinaryView**, **String** could also be **LargeString** or **StringView**, + and **List** could also be **LargeList** or **ListView**. + +The data would then be stored in Arrow as follows:: + + * Length: 4, Null count: 1 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00000111 | 0 (padding) | + + * Children arrays: + * field-0 array (`VarBinary` metadata) + * Length: 4, Null count: 0 + * Offsets buffer: + + | Bytes 0-19 | Bytes 20-63 | + |------------------|--------------------------| + | 0, 2, 4, 6, 8 | unspecified (padding) | + + * Value buffer: (01 00 -> indicates version 1 empty metadata) + + | Bytes 0-7 | Bytes 8-63 | + |-------------------------|--------------------------| + | 01 00 01 00 01 00 01 00 | unspecified (padding) | + + * field-1 array (`VarBinary` value) + * Length: 4, Null count: 1 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00001000 | 0 (padding) | + + * Offsets buffer: + + | Bytes 0-19 | Bytes 20-63 | + |------------------|--------------------------| + | 0, 0, 0, 0, 1 | unspecified (padding) | + + * Value buffer: (00 -> variant null) + + | Bytes 0 | Bytes 1-63 | + |--------------------|--------------------------| + | 00 | unspecified (padding) | + + * field-2 array (`List<Struct<VarBinary, String>>` typed_value) + * Length: 4, Null count: 1 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|-------------| + | 00000111 | 0 (padding) | + + * Offsets buffer (int32) + + | Bytes 0-19 | Bytes 20-63 | + |-------------------|-----------------------| + | 0, 2, 4, 7, 7 | unspecified (padding) | + + * Values array (`Struct<VarBinary, String>` element): + * Length: 7, Null count: 0 + * Validity bitmap buffer: Not required + + * Children arrays: + * field-0 array (`VarBinary` value) + * Length: 7, Null count: 6 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|-------------| + | 00001000 | 0 (padding) | + + * Offsets buffer (int32): + + | Bytes 0-31 | Bytes 32-63 | + |---------------------------|--------------------------| + | 0, 0, 0, 0, 1, 1, 1, 1 | unspecified (padding) | + + * Values buffer (`00` -> variant null): + + | Bytes 0 | Bytes 1-63 | + |--------------------|--------------------------| + | 00 | unspecified (padding) | + + * field-1 array (`String` typed_value) + * Length: 7, Null count: 1 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|-------------| + | 01110111 | 0 (padding) | + + * Offsets buffer (int32): + + | Bytes 0-31 | Bytes 32-63 | + |---------------------------------|--------------------------| + | 0, 6, 11, 17, 17, 23, 28, 35 | unspecified (padding) | + + * Values buffer: + + | Bytes 0-35 | Bytes 36-63 | + |--------------------------------------|--------------------------| + | comedydramahorrorcomedydramaromance | unspecified (padding) | + +Shredding an Object +''''''''''''''''''' + +Let's consider a JSON column of "events" which contain a field named ``event_type`` (a string) +and a field named ``event_ts`` (a timestamp) that we wish to shred into separate columns, In Parquet, +it could look something like this:: + + optional group event (VARIANT) { + required binary metadata; + optional binary value; # variant, remaining fields/values + optional group typed_value { # shredded fields for variant object + required group event_type { # event_type shredded field + optional binary value; + optional binary typed_value (STRING); + } + required group event_ts { # event_ts shredded field + optional binary value; + optional int64 typed_value (TIMESTAMP(true, MICROS)) + } + } + } + +We can then translate this into the expected extension storage type:: + + struct< + metadata: binary non-nullable, + value: binary nullable, + typed_value: struct< + event_type: struct< + value: binary nullable, + typed_value: string nullable + > non-nullable, + event_ts: struct< + value: binary nullable, + typed_value: timestamp(us, UTC) nullable + > non-nullable + > nullable + > + +If a field *does not exist* in the variant object value, then both the **value** and **typed_value** columns for that row +will be null. If a field is *present*, but the value is null, then **value** must contain a Variant null. + +It is *invalid* for both **value** and **typed_value** to be non-null for a given index. A reader can choose not to error +in this scenario, but if so it **must** use the value in the **typed_value** column for that index. + +Let's consider the following series of objects:: + + {"event_type": "noop", "event_ts": 1729794114937} + + {"event_type": "login", "event_ts": 1729794146402, "email": "[email protected]"} + + {"error_msg": "malformed..."} + + "malformed: not an object" + + {"event_ts": 1729794240241, "click": "_button"} + + {"event_ts": null, "event_ts": 1729794954163} + + {"event_type": "noop", "event_ts": "2024-10-24"} + + {} + + null + + *Entirely missing* + +To represent those values as a column of Variant values using the Variant extension type we get the following:: + + * Length: 10, Null count: 1 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Byte 1 | Bytes 2-63 | + |--------------------------|-----------|-----------------------| + | 11111111 | 00000001 | 0 (padding) | + + * Children arrays + * field-0 array (`VarBinary` Metadata) + * Length: 10, Null count: 0 + * Offsets buffer: + + | Bytes 0-43 (int32) | Bytes 44-63 | + |------------------------------------------|-------------------------| + | 0, 2, 11, 24, 26, 35, 37, 39, 41, 43, 45 | unspecified (padding) | + + * Value buffer: (01 00 -> version 1 empty metadata, + 01 01 00 XX ... -> Version 1, metadata with 1 elem, offset 0, offset XX == len(string), ... is dict string bytes) + + | Bytes 0-1 | Bytes 2-10 | Bytes 11-23 | Bytes 24-25 | Bytes 26-34 | + |-------------------------------|-----------------------|-------------|-------------------| + | 01 00 | 01 01 00 05 email | 01 01 00 09 error_msg | 01 00 | 01 01 00 05 click | + + | Bytes 35-36 | Bytes 37-38 | Bytes 39-40 | Bytes 41-42 | Bytes 43-44 | Bytes 45-63 | + |-------------|-------------|-------------|-------------|-------------|-----------------------| + | 01 00 | 01 00 | 01 00 | 01 00 | 01 00 | unspecified (padding) | + + * field-1 array (`VarBinary` Value) + * Length: 10, Null count: 5 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Byte 1 | Bytes 2-63 | + |---------------------------|-----------|-----------------------| + | 00011110 | 00000001 | 0 (padding) | + + * Offsets buffer (filled in based on lengths of encoded variants): + + | ... | + + * Value buffer: + + | VariantEncode({"email": "[email protected]"}) | VariantEncode({"error_msg": "malformed..."}) | + | VariantEncode("malformed: not an object") | VariantEncode({"click": "_button"}) | 00 (null) | + + * field-2 array (`Struct<...>` typed_value) + * Length: 10, Null count: 3 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Byte 1 | Bytes 2-63 | + |--------------------------|-----------|-----------------------| + | 11110111 | 00000000 | 0 (padding) | + + * Children arrays: + * field-0 array (`Struct<VarBinary, String>` event_type) + * Length: 10, Null count: 0 + * Validity bitmap buffer: not required + + * Children arrays + * field-0 array (`VarBinary` value) + * Length: 10, Null count: 9 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Byte 1 | Bytes 2-63 | + |--------------------------|-----------|-----------------------| + | 01000000 | 00000000 | 0 (padding) | + + * Offsets buffer (int32) + + | Bytes 0-43 (int32) | Bytes 44-63 | + |---------------------------------|-------------------------| + | 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1 | unspecified (padding) | + + * Value buffer: + + | Byte 0 | Bytes 1-63 | + |--------|------------------------| + | 00 | unspecified (padding) | + + * field-1 array (`String` typed_value) + * Length: 10, Null count: 7 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Byte 1 | Bytes 2-63 | + |--------------------------|-----------|-----------------------| + | 01000011 | 00000000 | 0 (padding) | + + * Offsets buffer (int32) + + | Byte 0-43 | Bytes 44-63 | + |-------------------------------------|------------------------| + | 0, 4, 9, 9, 9, 9, 9, 13, 13, 13, 13 | unspecified (padding) | + + * Value buffer: + + | Bytes 0-3 | Bytes 4-8 | Bytes 9-12 | Bytes 13-63 | + |-----------|-----------|------------|------------------------| + | noop | login | noop | unspecified (padding) | + + + * field-1 array (`Struct<VarBinary, Timestamp>` event_ts) + * Length: 10, Null count: 0 + * Validity bitmap buffer: not required + + * Children arrays + * field-0 array (`VarBinary` value) + * Length: 10, Null count: 9 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Byte 1 | Bytes 2-63 | + |--------------------------|-----------|-----------------------| + | 01000000 | 00000000 | 0 (padding) | + + * Offsets buffer (int32) + + | Bytes 0-43 (int32) | Bytes 44-63 | + |---------------------------------|-------------------------| + | ... | unspecified (padding) | + + * Value buffer: + + | VariantEncode("2024-10-24") | + + * field-1 array (`Timestamp(us, UTC)` typed_value) + * Length: 10, Null count: 6 + * Validity bitmap buffer: + + | Byte 0 (validity bitmap) | Byte 1 | Bytes 2-63 | + |--------------------------|-----------|-----------------------| + | 00110011 | 00000000 | 0 (padding) | + + * Value buffer: + + | Bytes 0-7 | Bytes 8-15 | Bytes 16-31 | Bytes 32-39 | Bytes 40-47 | Bytes 48-63 | + |---------------|---------------|--------------|---------------|---------------|------------------------| + | 1729794114937 | 1729794146402 | unspecified | 1729794240241 | 1729794954163 | unspecified (padding) | + + +Putting it all together +''''''''''''''''''''''' + +As mentioned, the **typed_value** field associated with a Variant **value** can be of any shredded type. As a result, +as long as we follow the original rules we can have an arbitrary number of nested levels based on how you want to +shred the object. For example, we might have a few more fields alongside **event_type** to shred out. Possibly an object +that looks like this:: + + { + "event_type": "login", + “event_ts”: 1729794114937, + “location”: { “longitude”: 1.5, “latitude”: 5.5 }, + “tags”: [“foo”, “bar”, “baz”] Review Comment: ```suggestion "event_ts": 1729794114937, "location”: {"longitude": 1.5, "latitude": 5.5}, "tags": ["foo", "bar", "baz"] ``` ########## docs/source/format/CanonicalExtensions.rst: ########## @@ -417,7 +421,591 @@ better zero-copy compatibility with various systems that also store booleans usi Metadata is an empty string. -========================= +.. _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: ``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 any :term:`primitive type` or a ``List``, ``LargeList``, ``ListView`` or ``Struct`` + + * If the ``typed_value`` field is a *nested* type, 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). +* 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``. + +.. note:: + + The fields may be in any order, and thus must be accessed by **name** not by *position*. The field names are case sensitive. + +Examples +-------- + +Unshredded +'''''''''' + +The simplest case, an unshredded variant always consists of **exactly** two fields: ``metadata`` and ``value``. Any of +the following storage types are valid (not an exhaustive list): + +* ``struct<metadata: binary non-nullable, value: binary nullable>`` +* ``struct<value: binary nullable, metadata: binary non-nullable>`` +* ``struct<metadata: dictionary<int8, binary> non-nullable, value: binary_view nullable>`` + +Simple Shredding +'''''''''''''''' + +Suppose we have a Variant field named *measurement* and we want to shred the ``int64`` values into a separate column for efficiency. +In Parquet, this could be represented as:: + + required group measurement (VARIANT) { + required binary metadata; + optional binary value; + optional int64 typed_value; + } + +Thus the corresponding storage type for the ``parquet.variant`` Arrow extension type would be:: + + struct< + metadata: binary required, + value: binary optional, + typed_value: int64 optional Review Comment: ```suggestion metadata: binary non-nullable, value: binary nullable, typed_value: int64 nullable ``` ########## docs/source/format/CanonicalExtensions.rst: ########## @@ -417,7 +421,591 @@ better zero-copy compatibility with various systems that also store booleans usi Metadata is an empty string. -========================= +.. _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: ``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 any :term:`primitive type` or a ``List``, ``LargeList``, ``ListView`` or ``Struct`` + + * If the ``typed_value`` field is a *nested* type, 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). +* 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``. + +.. note:: + + The fields may be in any order, and thus must be accessed by **name** not by *position*. The field names are case sensitive. + +Examples +-------- + +Unshredded +'''''''''' + +The simplest case, an unshredded variant always consists of **exactly** two fields: ``metadata`` and ``value``. Any of +the following storage types are valid (not an exhaustive list): + +* ``struct<metadata: binary non-nullable, value: binary nullable>`` +* ``struct<value: binary nullable, metadata: binary non-nullable>`` +* ``struct<metadata: dictionary<int8, binary> non-nullable, value: binary_view nullable>`` + +Simple Shredding +'''''''''''''''' + +Suppose we have a Variant field named *measurement* and we want to shred the ``int64`` values into a separate column for efficiency. +In Parquet, this could be represented as:: + + required group measurement (VARIANT) { + required binary metadata; + optional binary value; + optional int64 typed_value; + } + +Thus the corresponding storage type for the ``parquet.variant`` Arrow extension type would be:: + + struct< + metadata: binary required, + value: binary optional, + typed_value: int64 optional + > + +If we suppose a series of measurements consisting of:: + + 34, null, "n/a", 100 + +The data should be stored/represented in Arrow as:: + + * Length: 4, Null count: 1 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00001011 | 0 (padding) | + + * Children arrays: + * field-0 array (`VarBinary`) + * Length: 4, Null count: 0 + * Offsets buffer: + + | Bytes 0-19 | Bytes 20-63 | + |------------------|--------------------------| + | 0, 2, 4, 6, 8 | unspecified (padding) | + + * Value buffer: (01 00 -> indicates version 1 empty metadata) + + | Bytes 0-7 | Bytes 8-63 | + |-------------------------|--------------------------| + | 01 00 01 00 01 00 01 00 | unspecified (padding) | + + * field-1 array (`VarBinary`) + * Length: 4, Null count: 2 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00000110 | 0 (padding) | + + * Offsets buffer: + + | Bytes 0-19 | Bytes 20-63 | + |------------------|--------------------------| + | 0, 0, 1, 5, 5 | unspecified (padding) | + + * Value buffer: (`00` -> literal null, `0x13 0x6E 0x2F 0x61` -> variant encoding literal string "n/a") + + | Bytes 0-4 | Bytes 5-63 | + |------------------------|--------------------------| + | 00 0x13 0x6E 0x2F 0x61 | unspecified (padding) | + + * field-2 array (int64 array) + * Length: 4, Null count: 2 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00001001 | 0 (padding) | + + * Value buffer: + + | Bytes 0-31 | Bytes 32-63 | + |---------------------|--------------------------| + | 34, 00, 00, 100 | unspecified (padding) | + +.. note:: + + Notice that there is a variant ``literal null`` in the ``value`` array, this is due to the + `shredding specification <https://github.com/apache/parquet-format/blob/master/VariantShredding.md#value-shredding>`__ + so that a consumer can tell the difference between a *missing* field and a **null** field. A null + element must be encoded as a Variant null: *basic type* ``0`` (primitive) and *physical type* ``0`` (null). + +Shredding an Array +'''''''''''''''''' + +For our next example, we will represent a shredded array of strings. Let's consider a column that looks like: :: + + ["comedy", "drama"], ["horror", null], ["comedy", "drama", "romance"], null + +Representing this shredded variant in Parquet could look like:: + + optional group tags (VARIANT) { + required binary metadata; + optional binary value; + optional group typed_value (LIST) { # optional to allow null lists + repeated group list { + required group element { # shredded element + optional binary value; + optional binary typed_value (STRING); + } + } + } + } + +The array structure for Variant encoding does not allow missing elements, so all elements of the array must +be *non-nullable*. As such, either **typed_value** or **value** (*but not both!*) must be *non-null*. + +The storage type to represent this in Arrow as a Variant extension type would be:: + + struct< + metadata: binary non-nullable, + value: binary nullable, + typed_value: list<element: struct< + value: binary nullable, + typed_value: string nullable + > required> nullable + > + +.. note:: + + As usual, **Binary** could also be **LargeBinary** or **BinaryView**, **String** could also be **LargeString** or **StringView**, + and **List** could also be **LargeList** or **ListView**. Review Comment: ```suggestion As usual, **Binary** could also be **LargeBinary** or **BinaryView**, **String** could also be **LargeString** or **StringView**, and **List** could also be **LargeList** or **ListView**. ``` ########## docs/source/format/CanonicalExtensions.rst: ########## @@ -417,7 +421,591 @@ better zero-copy compatibility with various systems that also store booleans usi Metadata is an empty string. -========================= +.. _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: ``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 any :term:`primitive type` or a ``List``, ``LargeList``, ``ListView`` or ``Struct`` + + * If the ``typed_value`` field is a *nested* type, 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). +* 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``. + +.. note:: + + The fields may be in any order, and thus must be accessed by **name** not by *position*. The field names are case sensitive. + +Examples +-------- + +Unshredded +'''''''''' + +The simplest case, an unshredded variant always consists of **exactly** two fields: ``metadata`` and ``value``. Any of +the following storage types are valid (not an exhaustive list): + +* ``struct<metadata: binary non-nullable, value: binary nullable>`` +* ``struct<value: binary nullable, metadata: binary non-nullable>`` +* ``struct<metadata: dictionary<int8, binary> non-nullable, value: binary_view nullable>`` + +Simple Shredding +'''''''''''''''' + +Suppose we have a Variant field named *measurement* and we want to shred the ``int64`` values into a separate column for efficiency. +In Parquet, this could be represented as:: + + required group measurement (VARIANT) { + required binary metadata; + optional binary value; + optional int64 typed_value; + } + +Thus the corresponding storage type for the ``parquet.variant`` Arrow extension type would be:: + + struct< + metadata: binary required, + value: binary optional, + typed_value: int64 optional + > + +If we suppose a series of measurements consisting of:: + + 34, null, "n/a", 100 + +The data should be stored/represented in Arrow as:: + + * Length: 4, Null count: 1 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00001011 | 0 (padding) | + + * Children arrays: + * field-0 array (`VarBinary`) + * Length: 4, Null count: 0 + * Offsets buffer: + + | Bytes 0-19 | Bytes 20-63 | + |------------------|--------------------------| + | 0, 2, 4, 6, 8 | unspecified (padding) | + + * Value buffer: (01 00 -> indicates version 1 empty metadata) + + | Bytes 0-7 | Bytes 8-63 | + |-------------------------|--------------------------| + | 01 00 01 00 01 00 01 00 | unspecified (padding) | + + * field-1 array (`VarBinary`) + * Length: 4, Null count: 2 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00000110 | 0 (padding) | + + * Offsets buffer: + + | Bytes 0-19 | Bytes 20-63 | + |------------------|--------------------------| + | 0, 0, 1, 5, 5 | unspecified (padding) | + + * Value buffer: (`00` -> literal null, `0x13 0x6E 0x2F 0x61` -> variant encoding literal string "n/a") + + | Bytes 0-4 | Bytes 5-63 | + |------------------------|--------------------------| + | 00 0x13 0x6E 0x2F 0x61 | unspecified (padding) | + + * field-2 array (int64 array) + * Length: 4, Null count: 2 + * Validity Bitmap buffer: + + | Byte 0 (validity bitmap) | Bytes 1-63 | + |--------------------------|---------------| + | 00001001 | 0 (padding) | + + * Value buffer: + + | Bytes 0-31 | Bytes 32-63 | + |---------------------|--------------------------| + | 34, 00, 00, 100 | unspecified (padding) | + +.. note:: + + Notice that there is a variant ``literal null`` in the ``value`` array, this is due to the + `shredding specification <https://github.com/apache/parquet-format/blob/master/VariantShredding.md#value-shredding>`__ + so that a consumer can tell the difference between a *missing* field and a **null** field. A null + element must be encoded as a Variant null: *basic type* ``0`` (primitive) and *physical type* ``0`` (null). + +Shredding an Array +'''''''''''''''''' + +For our next example, we will represent a shredded array of strings. Let's consider a column that looks like: :: + + ["comedy", "drama"], ["horror", null], ["comedy", "drama", "romance"], null + +Representing this shredded variant in Parquet could look like:: + + optional group tags (VARIANT) { + required binary metadata; + optional binary value; + optional group typed_value (LIST) { # optional to allow null lists + repeated group list { + required group element { # shredded element + optional binary value; + optional binary typed_value (STRING); + } + } + } + } + +The array structure for Variant encoding does not allow missing elements, so all elements of the array must +be *non-nullable*. As such, either **typed_value** or **value** (*but not both!*) must be *non-null*. + +The storage type to represent this in Arrow as a Variant extension type would be:: + + struct< + metadata: binary non-nullable, + value: binary nullable, + typed_value: list<element: struct< + value: binary nullable, + typed_value: string nullable + > required> nullable Review Comment: ```suggestion > non-nullable> nullable ``` ########## docs/source/format/CanonicalExtensions.rst: ########## @@ -417,7 +421,591 @@ better zero-copy compatibility with various systems that also store booleans usi Metadata is an empty string. -========================= +.. _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: ``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 any :term:`primitive type` or a ``List``, ``LargeList``, ``ListView`` or ``Struct`` + + * If the ``typed_value`` field is a *nested* type, 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). +* Extension type parameters: Review Comment: ```suggestion * Extension type parameters: ``` -- This is an automated message from the Apache Git Service. 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