datafusion_functions/core/
coalesce.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::{new_null_array, BooleanArray};
use arrow::compute::kernels::zip::zip;
use arrow::compute::{and, is_not_null, is_null};
use arrow::datatypes::DataType;
use datafusion_common::{exec_err, ExprSchema, Result};
use datafusion_expr::binary::try_type_union_resolution;
use datafusion_expr::scalar_doc_sections::DOC_SECTION_CONDITIONAL;
use datafusion_expr::{ColumnarValue, Documentation, Expr, ExprSchemable};
use datafusion_expr::{ScalarUDFImpl, Signature, Volatility};
use itertools::Itertools;
use std::any::Any;
use std::sync::OnceLock;

#[derive(Debug)]
pub struct CoalesceFunc {
    signature: Signature,
}

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

impl CoalesceFunc {
    pub fn new() -> Self {
        Self {
            signature: Signature::user_defined(Volatility::Immutable),
        }
    }
}

impl ScalarUDFImpl for CoalesceFunc {
    fn as_any(&self) -> &dyn Any {
        self
    }

    fn name(&self) -> &str {
        "coalesce"
    }

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

    fn return_type(&self, arg_types: &[DataType]) -> Result<DataType> {
        Ok(arg_types
            .iter()
            .find_or_first(|d| !d.is_null())
            .unwrap()
            .clone())
    }

    // If any the arguments in coalesce is non-null, the result is non-null
    fn is_nullable(&self, args: &[Expr], schema: &dyn ExprSchema) -> bool {
        args.iter().all(|e| e.nullable(schema).ok().unwrap_or(true))
    }

    /// coalesce evaluates to the first value which is not NULL
    fn invoke(&self, args: &[ColumnarValue]) -> Result<ColumnarValue> {
        // do not accept 0 arguments.
        if args.is_empty() {
            return exec_err!(
                "coalesce was called with {} arguments. It requires at least 1.",
                args.len()
            );
        }

        let return_type = args[0].data_type();
        let mut return_array = args.iter().filter_map(|x| match x {
            ColumnarValue::Array(array) => Some(array.len()),
            _ => None,
        });

        if let Some(size) = return_array.next() {
            // start with nulls as default output
            let mut current_value = new_null_array(&return_type, size);
            let mut remainder = BooleanArray::from(vec![true; size]);

            for arg in args {
                match arg {
                    ColumnarValue::Array(ref array) => {
                        let to_apply = and(&remainder, &is_not_null(array.as_ref())?)?;
                        current_value = zip(&to_apply, array, &current_value)?;
                        remainder = and(&remainder, &is_null(array)?)?;
                    }
                    ColumnarValue::Scalar(value) => {
                        if value.is_null() {
                            continue;
                        } else {
                            let last_value = value.to_scalar()?;
                            current_value = zip(&remainder, &last_value, &current_value)?;
                            break;
                        }
                    }
                }
                if remainder.iter().all(|x| x == Some(false)) {
                    break;
                }
            }
            Ok(ColumnarValue::Array(current_value))
        } else {
            let result = args
                .iter()
                .filter_map(|x| match x {
                    ColumnarValue::Scalar(s) if !s.is_null() => Some(x.clone()),
                    _ => None,
                })
                .next()
                .unwrap_or_else(|| args[0].clone());
            Ok(result)
        }
    }

    fn short_circuits(&self) -> bool {
        true
    }

    fn coerce_types(&self, arg_types: &[DataType]) -> Result<Vec<DataType>> {
        if arg_types.is_empty() {
            return exec_err!("coalesce must have at least one argument");
        }

        try_type_union_resolution(arg_types)
    }

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

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

fn get_coalesce_doc() -> &'static Documentation {
    DOCUMENTATION.get_or_init(|| {
        Documentation::builder()
            .with_doc_section(DOC_SECTION_CONDITIONAL)
            .with_description("Returns the first of its arguments that is not _null_. Returns _null_ if all arguments are _null_. This function is often used to substitute a default value for _null_ values.")
            .with_syntax_example("coalesce(expression1[, ..., expression_n])")
            .with_sql_example(r#"```sql
> select coalesce(null, null, 'datafusion');
+----------------------------------------+
| coalesce(NULL,NULL,Utf8("datafusion")) |
+----------------------------------------+
| datafusion                             |
+----------------------------------------+
```"#,
            )
            .with_argument(
                "expression1, expression_n",
                "Expression to use if previous expressions are _null_. Can be a constant, column, or function, and any combination of arithmetic operators. Pass as many expression arguments as necessary."
            )
            .build()
            .unwrap()
    })
}

#[cfg(test)]
mod test {
    use arrow::datatypes::DataType;

    use datafusion_expr::ScalarUDFImpl;

    use crate::core;

    #[test]
    fn test_coalesce_return_types() {
        let coalesce = core::coalesce::CoalesceFunc::new();
        let return_type = coalesce
            .return_type(&[DataType::Date32, DataType::Date32])
            .unwrap();
        assert_eq!(return_type, DataType::Date32);
    }

    #[test]
    fn test_coalesce_return_types_with_nulls_first() {
        let coalesce = core::coalesce::CoalesceFunc::new();
        let return_type = coalesce
            .return_type(&[DataType::Null, DataType::Date32])
            .unwrap();
        assert_eq!(return_type, DataType::Date32);
    }

    #[test]
    fn test_coalesce_return_types_with_nulls_last() {
        let coalesce = core::coalesce::CoalesceFunc::new();
        let return_type = coalesce
            .return_type(&[DataType::Int64, DataType::Null])
            .unwrap();
        assert_eq!(return_type, DataType::Int64);
    }
}
datafusion_functions/core/coalesce.rs

datafusion_functions/core/
coalesce.rs
