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.. _format_canonical_extension_examples:
****************************
Canonical Extension Examples
****************************
=========================
Parquet Variant Extension
=========================
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 ``arrow.parquet.variant`` Arrow extension type would be::
struct<
metadata: binary non-nullable,
value: binary nullable,
typed_value: int64 nullable
>
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` -> 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
> non-nullable> 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": "user@example.com"}
{"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": "user@email.com"}) | 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"]
}
If we shred the extra fields out and represent it as Parquet it looks like::
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))
}
required group location { # location shredded field
optional binary value;
optional group typed_value {
required group longitude {
optional binary value;
optional float64 typed_value;
}
required group latitude {
optional binary value;
optional float64 typed_value;
}
}
}
required group tags { # tags shredded field
optional binary value;
optional group typed_value (LIST) {
repeated group list {
required group element {
optional binary value;
optional binary typed_value (STRING);
}
}
}
}
}
}
Finally, following the rules we set forth on constructing the Variant Extension Type storage type, we end up with::
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,
location: struct<
value: binary nullable,
typed_value: struct<
longitude: struct<value: binary nullable, typed_value: double nullable> non-nullable,
latitude: struct<value: binary nullable, typed_value: double nullable> non-nullable
> nullable> non-nullable,
tags: struct<
value: binary nullable,
typed_value: list<struct<value: binary nullable, typed_value: string nullable> non-nullable> nullable
> non-nullable
> nullable
>