alamb commented on code in PR #7404:
URL: https://github.com/apache/arrow-rs/pull/7404#discussion_r2039831129
##########
parquet/src/arrow/arrow_reader/mod.rs:
##########
@@ -4431,4 +4431,258 @@ mod tests {
assert_eq!(c0.len(), c1.len());
c0.iter().zip(c1.iter()).for_each(|(l, r)| assert_eq!(l, r));
}
+
+ #[test]
+ #[cfg(feature = "arrow_canonical_extension_types")]
Review Comment:
one of the benefits of using the existing StructArray mechanism rather than
a new Array type is that we can likely reuse most of the existing parquet
writing code, focusing on ensuring the metadata is written/read accurately
##########
arrow-schema/src/extension/canonical/variant.rs:
##########
@@ -0,0 +1,233 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! Variant
+//!
+//! <https://arrow.apache.org/docs/format/CanonicalExtensions.html#variant>
+
+use base64::engine::Engine as _;
+use base64::engine::general_purpose::STANDARD;
+use crate::{extension::ExtensionType, ArrowError, DataType};
+
+/// The extension type for `Variant`.
+///
+/// Extension name: `arrow.variant`.
+///
+/// The storage type of this extension is **Binary or LargeBinary**.
+/// A Variant is a flexible structure that can store **Primitives, Arrays, or
Objects**.
+/// It is stored as **two binary values**: `metadata` and `value`.
+///
+/// The **metadata field is required** and must be a valid Variant metadata
string.
+/// The **value field is required** and contains the serialized Variant data.
+///
+/// <https://arrow.apache.org/docs/format/CanonicalExtensions.html#variant>
+#[derive(Debug, Clone, PartialEq)]
+pub struct Variant {
+ metadata: Vec<u8>, // Required binary metadata
+ value: Vec<u8>, // Required binary value
+}
+
+impl Variant {
+ /// Creates a new `Variant` with metadata and value.
+ pub fn new(metadata: Vec<u8>, value: Vec<u8>) -> Self {
+ Self { metadata, value }
+ }
+
+ /// Creates a Variant representing an empty structure.
+ pub fn empty() -> Result<Self, ArrowError> {
+ Err(ArrowError::InvalidArgumentError(
+ "Variant cannot be empty because metadata and value are
required".to_owned(),
+ ))
+ }
+
+ /// Returns the metadata as a byte array.
+ pub fn metadata(&self) -> &[u8] {
+ &self.metadata
+ }
+
+ /// Returns the value as an byte array.
+ pub fn value(&self) -> &[u8] {
+ &self.value
+ }
+
+ /// Sets the value of the Variant.
+ pub fn set_value(mut self, value: Vec<u8>) -> Self {
+ self.value = value;
+ self
+ }
+}
+
+impl ExtensionType for Variant {
+ const NAME: &'static str = "arrow.variant";
+
+ type Metadata = Vec<u8>; // Metadata is directly Vec<u8>
+
+ fn metadata(&self) -> &Self::Metadata {
+ &self.metadata
+ }
+
+ fn serialize_metadata(&self) -> Option<String> {
+ Some(STANDARD.encode(&self.metadata)) // Encode metadata as STANDARD
string
Review Comment:
My understanding of the extension metadata for a variant column isn't the
same as the metadata for each variant value -- I probably don't fully
understand the code but I think the extension type metadata is stored for *each
field* where the variant's metadata is stored for each *row* (basically the
word metadata is overloaded 😢 )
##########
arrow-schema/src/extension/canonical/variant.rs:
##########
@@ -0,0 +1,233 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! Variant
+//!
+//! <https://arrow.apache.org/docs/format/CanonicalExtensions.html#variant>
+
+use base64::engine::Engine as _;
+use base64::engine::general_purpose::STANDARD;
+use crate::{extension::ExtensionType, ArrowError, DataType};
+
+/// The extension type for `Variant`.
+///
+/// Extension name: `arrow.variant`.
+///
+/// The storage type of this extension is **Binary or LargeBinary**.
+/// A Variant is a flexible structure that can store **Primitives, Arrays, or
Objects**.
+/// It is stored as **two binary values**: `metadata` and `value`.
+///
+/// The **metadata field is required** and must be a valid Variant metadata
string.
+/// The **value field is required** and contains the serialized Variant data.
+///
+/// <https://arrow.apache.org/docs/format/CanonicalExtensions.html#variant>
+#[derive(Debug, Clone, PartialEq)]
+pub struct Variant {
+ metadata: Vec<u8>, // Required binary metadata
Review Comment:
One concern I have with this approach is that it will *require* two
allocations (and memory copies) to read a Variant value
I am hoping we can use Rust's borrow checker to do this with pointers (well,
slices in rust) and no copying -- something like
```rust
pub struct Variant<'a> {
metadata: &'a [u8], // Required binary metadata
value: &'a [u8],
}
```
##########
arrow-array/src/array/variant_array.rs:
##########
@@ -0,0 +1,628 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use crate::array::print_long_array;
+use crate::builder::{ArrayBuilder, BinaryBuilder};
+use crate::{Array, ArrayRef};
+use arrow_buffer::{Buffer, NullBuffer, OffsetBuffer, ScalarBuffer};
+use arrow_data::{ArrayData, ArrayDataBuilder};
+use arrow_schema::{ArrowError, DataType, Field};
+
+#[cfg(feature = "canonical_extension_types")]
+use arrow_schema::extension::Variant;
+use std::sync::Arc;
+use std::any::Any;
+
+/// An array of Variant values.
+///
+/// The Variant extension type stores data as two binary values: metadata and
value.
+/// This array stores each Variant as a concatenated binary value (metadata +
value).
+///
+/// # Example
+///
+/// ```
+/// use arrow_array::VariantArray;
+/// use arrow_schema::extension::Variant;
+/// use arrow_array::Array; // Import the Array trait
+///
+/// // Create metadata and value for each variant
+/// let metadata = vec![
+/// 0x01, // header: version=1, sorted=0, offset_size=1
+/// 0x01, // dictionary_size = 1
+/// 0x00, // offset 0
+/// 0x03, // offset 3
+/// b'k', b'e', b'y' // dictionary bytes
+/// ];
+/// let variant_type = Variant::new(metadata.clone(), vec![]);
+///
+/// // Create variants with different values
+/// let variants = vec![
+/// Variant::new(metadata.clone(), b"null".to_vec()),
+/// Variant::new(metadata.clone(), b"true".to_vec()),
+/// Variant::new(metadata.clone(), b"{\"a\": 1}".to_vec()),
+/// ];
+///
+/// // Create a VariantArray
+/// let variant_array = VariantArray::from_variants(variant_type,
variants.clone()).expect("Failed to create VariantArray");
+///
+/// // Access variants from the array
+/// assert_eq!(variant_array.len(), 3);
+/// let retrieved = variant_array.value(0).expect("Failed to get value");
+/// assert_eq!(retrieved.metadata(), &metadata);
+/// assert_eq!(retrieved.value(), b"null");
+/// ```
+#[cfg(feature = "canonical_extension_types")]
+pub mod variant_array_module {
+ use super::*;
+
+ /// An array of Variant values.
+ ///
+ /// The Variant extension type stores data as two binary values: metadata
and value.
+ /// This array stores each Variant as a concatenated binary value
(metadata + value).
+ ///
+ /// # Example
+ ///
+ /// ```
+ /// use arrow_array::VariantArray;
+ /// use arrow_schema::extension::Variant;
+ /// use arrow_array::Array; // Import the Array trait
+ ///
+ /// // Create metadata and value for each variant
+ /// let metadata = vec![
+ /// 0x01, // header: version=1, sorted=0, offset_size=1
+ /// 0x01, // dictionary_size = 1
+ /// 0x00, // offset 0
+ /// 0x03, // offset 3
+ /// b'k', b'e', b'y' // dictionary bytes
+ /// ];
+ /// let variant_type = Variant::new(metadata.clone(), vec![]);
+ ///
+ /// // Create variants with different values
+ /// let variants = vec![
+ /// Variant::new(metadata.clone(), b"null".to_vec()),
+ /// Variant::new(metadata.clone(), b"true".to_vec()),
+ /// Variant::new(metadata.clone(), b"{\"a\": 1}".to_vec()),
+ /// ];
+ ///
+ /// // Create a VariantArray
+ /// let variant_array = VariantArray::from_variants(variant_type,
variants.clone()).expect("Failed to create VariantArray");
+ ///
+ /// // Access variants from the array
+ /// assert_eq!(variant_array.len(), 3);
+ /// let retrieved = variant_array.value(0).expect("Failed to get value");
+ /// assert_eq!(retrieved.metadata(), &metadata);
+ /// assert_eq!(retrieved.value(), b"null");
+ /// ```
+ #[derive(Clone, Debug)]
+ pub struct VariantArray {
Review Comment:
One thing that we may want to explore is to model this as a `StructArray`
with two fields, `"metadata"` and `"value"` which appears to be how @neilechao
modeled it in the C++ implementation that merged a few days ago
- https://github.com/apache/arrow/pull/45375
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