datafusion_functions/core/
struct.rs

// 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 arrow::array::{ArrayRef, StructArray};
use arrow::datatypes::{DataType, Field, Fields};
use datafusion_common::{exec_err, Result};
use datafusion_expr::scalar_doc_sections::DOC_SECTION_STRUCT;
use datafusion_expr::{ColumnarValue, Documentation};
use datafusion_expr::{ScalarUDFImpl, Signature, Volatility};
use std::any::Any;
use std::sync::{Arc, OnceLock};

fn array_struct(args: &[ArrayRef]) -> Result<ArrayRef> {
    // do not accept 0 arguments.
    if args.is_empty() {
        return exec_err!("struct requires at least one argument");
    }

    let fields = args
        .iter()
        .enumerate()
        .map(|(i, arg)| {
            let field_name = format!("c{i}");
            Ok(Arc::new(Field::new(
                field_name.as_str(),
                arg.data_type().clone(),
                true,
            )))
        })
        .collect::<Result<Vec<_>>>()?
        .into();

    let arrays = args.to_vec();

    Ok(Arc::new(StructArray::new(fields, arrays, None)))
}

/// put values in a struct array.
fn struct_expr(args: &[ColumnarValue]) -> Result<ColumnarValue> {
    let arrays = ColumnarValue::values_to_arrays(args)?;
    Ok(ColumnarValue::Array(array_struct(arrays.as_slice())?))
}

#[derive(Debug)]
pub struct StructFunc {
    signature: Signature,
    aliases: Vec<String>,
}

impl Default for StructFunc {
    fn default() -> Self {
        Self::new()
    }
}

impl StructFunc {
    pub fn new() -> Self {
        Self {
            signature: Signature::variadic_any(Volatility::Immutable),
            aliases: vec![String::from("row")],
        }
    }
}

impl ScalarUDFImpl for StructFunc {
    fn as_any(&self) -> &dyn Any {
        self
    }
    fn name(&self) -> &str {
        "struct"
    }

    fn aliases(&self) -> &[String] {
        &self.aliases
    }

    fn signature(&self) -> &Signature {
        &self.signature
    }

    fn return_type(&self, arg_types: &[DataType]) -> Result<DataType> {
        let return_fields = arg_types
            .iter()
            .enumerate()
            .map(|(pos, dt)| Field::new(format!("c{pos}"), dt.clone(), true))
            .collect::<Vec<Field>>();
        Ok(DataType::Struct(Fields::from(return_fields)))
    }

    fn invoke(&self, args: &[ColumnarValue]) -> Result<ColumnarValue> {
        struct_expr(args)
    }

    fn documentation(&self) -> Option<&Documentation> {
        Some(get_struct_doc())
    }
}

static DOCUMENTATION: OnceLock<Documentation> = OnceLock::new();

fn get_struct_doc() -> &'static Documentation {
    DOCUMENTATION.get_or_init(|| {
        Documentation::builder()
            .with_doc_section(DOC_SECTION_STRUCT)
            .with_description("Returns an Arrow struct using the specified input expressions optionally named.
Fields in the returned struct use the optional name or the `cN` naming convention.
For example: `c0`, `c1`, `c2`, etc.")
            .with_syntax_example("struct(expression1[, ..., expression_n])")
            .with_sql_example(r#"For example, this query converts two columns `a` and `b` to a single column with
a struct type of fields `field_a` and `c1`:
```sql
> select * from t;
+---+---+
| a | b |
+---+---+
| 1 | 2 |
| 3 | 4 |
+---+---+

-- use default names `c0`, `c1`
> select struct(a, b) from t;
+-----------------+
| struct(t.a,t.b) |
+-----------------+
| {c0: 1, c1: 2}  |
| {c0: 3, c1: 4}  |
+-----------------+

-- name the first field `field_a`
select struct(a as field_a, b) from t;
+--------------------------------------------------+
| named_struct(Utf8("field_a"),t.a,Utf8("c1"),t.b) |
+--------------------------------------------------+
| {field_a: 1, c1: 2}                              |
| {field_a: 3, c1: 4}                              |
+--------------------------------------------------+
```
"#)
            .with_argument(
                "expression1, expression_n",
                "Expression to include in the output struct. Can be a constant, column, or function, any combination of arithmetic or string operators.")
            .build()
            .unwrap()
    })
}
datafusion_functions/core/struct.rs

datafusion_functions/core/
struct.rs
