apache · hareshkh · Oct 15, 2025
diff --git a/datafusion/spark/src/function/string/concat.rs b/datafusion/spark/src/function/string/concat.rs
@@ -0,0 +1,306 @@
+// 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::{Array, ArrayBuilder};
+use arrow::datatypes::DataType;
+use datafusion_common::{Result, ScalarValue};
+use datafusion_expr::{
+    ColumnarValue, ScalarFunctionArgs, ScalarUDFImpl, Signature, TypeSignature,
+    Volatility,
+};
+use datafusion_functions::string::concat::ConcatFunc;
+use std::any::Any;
+use std::sync::Arc;
+
+/// Spark-compatible `concat` expression
+/// <https://spark.apache.org/docs/latest/api/sql/index.html#concat>
+///
+/// Concatenates multiple input strings into a single string.
+/// Returns NULL if any input is NULL.
+#[derive(Debug, PartialEq, Eq, Hash)]
+pub struct SparkConcat {
+    signature: Signature,
+}
+
+impl Default for SparkConcat {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl SparkConcat {
+    pub fn new() -> Self {
+        Self {
+            signature: Signature::one_of(
+                vec![TypeSignature::UserDefined, TypeSignature::Nullary],
+                Volatility::Immutable,
+            ),
+        }
+    }
+}
+
+impl ScalarUDFImpl for SparkConcat {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn name(&self) -> &str {
+        "concat"
+    }
+
+    fn signature(&self) -> &Signature {
+        &self.signature
+    }
+
+    fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
+        Ok(DataType::Utf8)
+    }
+
+    fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
+        spark_concat(args)
+    }
+
+    fn coerce_types(&self, arg_types: &[DataType]) -> Result<Vec<DataType>> {
+        // Accept any string types, including zero arguments
+        Ok(arg_types.to_vec())
+    }
+}
+
+/// Concatenates strings, returning NULL if any input is NULL
+/// This is a Spark-specific wrapper around DataFusion's concat that returns NULL
+/// if any argument is NULL (Spark behavior), whereas DataFusion's concat ignores NULLs.
+fn spark_concat(args: ScalarFunctionArgs) -> Result<ColumnarValue> {
+    let ScalarFunctionArgs {
+        args: arg_values,
+        arg_fields,
+        number_rows,
+        return_field,
+        config_options,
+    } = args;
+
+    // Handle zero-argument case: return empty string
+    if arg_values.is_empty() {
+        return Ok(ColumnarValue::Scalar(ScalarValue::Utf8(
+            Some(String::new()),
+        )));
+    }
+
+    // Step 1: Check for NULL mask in incoming args
+    let null_mask = compute_null_mask(&arg_values, number_rows)?;
+
+    // If all scalars and any is NULL, return NULL immediately
+    if null_mask.is_none() {
+        return Ok(ColumnarValue::Scalar(ScalarValue::Utf8(None)));
+    }
+
+    // Step 2: Delegate to DataFusion's concat
+    let concat_func = ConcatFunc::new();
+    let func_args = ScalarFunctionArgs {
+        args: arg_values,
+        arg_fields,
+        number_rows,
+        return_field,
+        config_options,
+    };
+    let result = concat_func.invoke_with_args(func_args)?;
+
+    // Step 3: Apply NULL mask to result
+    apply_null_mask(result, null_mask)
+}
+
+/// Compute NULL mask for the arguments
+/// Returns None if all scalars and any is NULL, or a Vector of
+/// boolean representing the null mask for incoming arrays
+fn compute_null_mask(
+    args: &[ColumnarValue],
+    number_rows: usize,
+) -> Result<Option<Vec<bool>>> {
+    // Check if all arguments are scalars
+    let all_scalars = args
+        .iter()
+        .all(|arg| matches!(arg, ColumnarValue::Scalar(_)));
+
+    if all_scalars {
+        // For scalars, check if any is NULL
+        for arg in args {
+            if let ColumnarValue::Scalar(scalar) = arg {
+                if scalar.is_null() {
+                    // Return None to indicate all values should be NULL
+                    return Ok(None);
+                }
+            }
+        }
+        // No NULLs in scalars
+        Ok(Some(vec![]))
+    } else {
+        // For arrays, compute NULL mask for each row
+        let array_len = args
+            .iter()
+            .find_map(|arg| match arg {
+                ColumnarValue::Array(array) => Some(array.len()),
+                _ => None,
+            })
+            .unwrap_or(number_rows);
+
+        // Convert all scalars to arrays for uniform processing
+        let arrays: Result<Vec<_>> = args
+            .iter()
+            .map(|arg| match arg {
+                ColumnarValue::Array(array) => Ok(Arc::clone(array)),
+                ColumnarValue::Scalar(scalar) => scalar.to_array_of_size(array_len),
+            })
+            .collect();
+        let arrays = arrays?;
+
+        // Compute NULL mask
+        let mut null_mask = vec![false; array_len];
+        for array in &arrays {
+            for (i, null_flag) in null_mask.iter_mut().enumerate().take(array_len) {
+                if array.is_null(i) {
+                    *null_flag = true;
+                }
+            }
+        }
+
+        Ok(Some(null_mask))
+    }
+}
+
+/// Apply NULL mask to the result
+fn apply_null_mask(
+    result: ColumnarValue,
+    null_mask: Option<Vec<bool>>,
+) -> Result<ColumnarValue> {
+    match (result, null_mask) {
+        // Scalar with NULL mask means return NULL
+        (ColumnarValue::Scalar(_), None) => {
+            Ok(ColumnarValue::Scalar(ScalarValue::Utf8(None)))
+        }
+        // Scalar without NULL mask, return as-is
+        (scalar @ ColumnarValue::Scalar(_), Some(mask)) if mask.is_empty() => Ok(scalar),
+        // Array with NULL mask
+        (ColumnarValue::Array(array), Some(null_mask)) if !null_mask.is_empty() => {
+            let array_len = array.len();
+            let return_type = array.data_type();
+
+            let mut builder: Box<dyn ArrayBuilder> = match return_type {
+                DataType::Utf8 => {
+                    let string_array = array
+                        .as_any()
+                        .downcast_ref::<arrow::array::StringArray>()
+                        .unwrap();
+                    let mut builder =
+                        arrow::array::StringBuilder::with_capacity(array_len, 0);
+                    for (i, &is_null) in null_mask.iter().enumerate().take(array_len) {
+                        if is_null || string_array.is_null(i) {
+                            builder.append_null();
+                        } else {
+                            builder.append_value(string_array.value(i));
+                        }
+                    }
+                    Box::new(builder)
+                }
+                DataType::LargeUtf8 => {
+                    let string_array = array
+                        .as_any()
+                        .downcast_ref::<arrow::array::LargeStringArray>()
+                        .unwrap();
+                    let mut builder =
+                        arrow::array::LargeStringBuilder::with_capacity(array_len, 0);
+                    for (i, &is_null) in null_mask.iter().enumerate().take(array_len) {
+                        if is_null || string_array.is_null(i) {
+                            builder.append_null();
+                        } else {
+                            builder.append_value(string_array.value(i));
+                        }
+                    }
+                    Box::new(builder)
+                }
+                DataType::Utf8View => {
+                    let string_array = array
+                        .as_any()
+                        .downcast_ref::<arrow::array::StringViewArray>()
+                        .unwrap();
+                    let mut builder =
+                        arrow::array::StringViewBuilder::with_capacity(array_len);
+                    for (i, &is_null) in null_mask.iter().enumerate().take(array_len) {
+                        if is_null || string_array.is_null(i) {
+                            builder.append_null();
+                        } else {
+                            builder.append_value(string_array.value(i));
+                        }
+                    }
+                    Box::new(builder)
+                }
+                _ => {
+                    return datafusion_common::exec_err!(
+                        "Unsupported return type for concat: {:?}",
+                        return_type
+                    );
+                }
+            };
+
+            Ok(ColumnarValue::Array(builder.finish()))
+        }
+        // Array without NULL mask, return as-is
+        (array @ ColumnarValue::Array(_), _) => Ok(array),
+        // Shouldn't happen
+        (scalar, _) => Ok(scalar),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::function::utils::test::test_scalar_function;
+    use arrow::array::StringArray;
+    use arrow::datatypes::DataType;
+    use datafusion_common::Result;
+
+    #[test]
+    fn test_concat_basic() -> Result<()> {
+        test_scalar_function!(
+            SparkConcat::new(),
+            vec![
+                ColumnarValue::Scalar(ScalarValue::Utf8(Some("Spark".to_string()))),
+                ColumnarValue::Scalar(ScalarValue::Utf8(Some("SQL".to_string()))),
+            ],
+            Ok(Some("SparkSQL")),
+            &str,
+            DataType::Utf8,
+            StringArray
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn test_concat_with_null() -> Result<()> {
+        test_scalar_function!(
+            SparkConcat::new(),
+            vec![
+                ColumnarValue::Scalar(ScalarValue::Utf8(Some("Spark".to_string()))),
+                ColumnarValue::Scalar(ScalarValue::Utf8(Some("SQL".to_string()))),
+                ColumnarValue::Scalar(ScalarValue::Utf8(None)),
+            ],
+            Ok(None),
+            &str,
+            DataType::Utf8,
+            StringArray
+        );
+        Ok(())
+    }
+}
diff --git a/datafusion/spark/src/function/string/mod.rs b/datafusion/spark/src/function/string/mod.rs
@@ -17,6 +17,7 @@
 
 pub mod ascii;
 pub mod char;
+pub mod concat;
 pub mod ilike;
 pub mod like;
 pub mod luhn_check;
@@ -27,6 +28,7 @@ use std::sync::Arc;
 
 make_udf_function!(ascii::SparkAscii, ascii);
 make_udf_function!(char::CharFunc, char);
+make_udf_function!(concat::SparkConcat, concat);
 make_udf_function!(ilike::SparkILike, ilike);
 make_udf_function!(like::SparkLike, like);
 make_udf_function!(luhn_check::SparkLuhnCheck, luhn_check);
@@ -44,6 +46,11 @@ pub mod expr_fn {
         "Returns the ASCII character having the binary equivalent to col. If col is larger than 256 the result is equivalent to char(col % 256).",
         arg1
     ));
+    export_functions!((
+        concat,
+        "Concatenates multiple input strings into a single string. Returns NULL if any input is NULL.",
+        args
+    ));
     export_functions!((
         ilike,
         "Returns true if str matches pattern (case insensitive).",
@@ -62,5 +69,5 @@ pub mod expr_fn {
 }
 
 pub fn functions() -> Vec<Arc<ScalarUDF>> {
-    vec![ascii(), char(), ilike(), like(), luhn_check()]
+    vec![ascii(), char(), concat(), ilike(), like(), luhn_check()]
 }
diff --git a/datafusion/sqllogictest/test_files/spark/string/concat.slt b/datafusion/sqllogictest/test_files/spark/string/concat.slt
@@ -0,0 +1,48 @@
+# 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.
+
+query T
+SELECT concat('Spark', 'SQL');
+----
+SparkSQL
+
+query T
+SELECT concat('Spark', 'SQL', NULL);
+----
+NULL
+
+query T
+SELECT concat('', '1', '', '2');
+----
+12
+
+query T
+SELECT concat();
+----
+(empty)
+
+query T
+SELECT concat('');
+----
+(empty)
+
+
+query T
+SELECT concat(a, b, c) from (select 'a' a, 'b' b, 'c' c union all select null a, 'b', 'c') order by 1 nulls last;
+----
+abc
+NULL