feat(function): add greatest function

datafusion/functions/src/core/greatest.rs

@@ -0,0 +1,272 @@
+// 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::{make_comparator, Array, ArrayRef, BooleanArray};
+use arrow::compute::kernels::cmp;
+use arrow::compute::kernels::zip::zip;
+use arrow::compute::SortOptions;
+use arrow::datatypes::DataType;
+use arrow_buffer::BooleanBuffer;
+use datafusion_common::{exec_err, plan_err, Result, ScalarValue};
+use datafusion_expr::binary::type_union_resolution;
+use datafusion_expr::scalar_doc_sections::DOC_SECTION_CONDITIONAL;
+use datafusion_expr::{ColumnarValue, Documentation};
+use datafusion_expr::{ScalarUDFImpl, Signature, Volatility};
+use std::any::Any;
+use std::sync::{Arc, OnceLock};
+
+const SORT_OPTIONS: SortOptions = SortOptions {
+    // We want greatest first
+    descending: false,
+
+    // NULL will be less than any other value
+    nulls_first: true,
+};
+
+#[derive(Debug)]
+pub struct GreatestFunc {
+    signature: Signature,
+}
+
+impl Default for GreatestFunc {
+    fn default() -> Self {
+        GreatestFunc::new()
+    }
+}
+
+impl GreatestFunc {
+    pub fn new() -> Self {
+        Self {
+            signature: Signature::user_defined(Volatility::Immutable),
+        }
+    }
+}
+
+fn get_logical_null_count(arr: &dyn Array) -> usize {
+    arr.logical_nulls()
+        .map(|n| n.null_count())
+        .unwrap_or_default()
+}
+
+/// Return boolean array where `arr[i] = lhs[i] >= rhs[i]` for all i, where `arr` is the result array
+/// Nulls are always considered smaller than any other value
+fn get_larger(lhs: &dyn Array, rhs: &dyn Array) -> Result<BooleanArray> {
+    // Fast path:
+    // If both arrays are not nested, have the same length and no nulls, we can use the faster vectorised kernel
+    // - If both arrays are not nested: Nested types, such as lists, are not supported as the null semantics are not well-defined.
+    // - both array does not have any nulls: cmp::gt_eq will return null if any of the input is null while we want to return false in that case
+    if !lhs.data_type().is_nested()
+        && get_logical_null_count(lhs) == 0
+        && get_logical_null_count(rhs) == 0
+    {
+        return cmp::gt_eq(&lhs, &rhs).map_err(|e| e.into());
+    }
+
+    let cmp = make_comparator(lhs, rhs, SORT_OPTIONS)?;
+
+    if lhs.len() != rhs.len() {
+        return exec_err!(
+            "All arrays should have the same length for greatest comparison"
+        );
+    }
+
+    let values = BooleanBuffer::collect_bool(lhs.len(), |i| cmp(i, i).is_ge());
+
+    // No nulls as we only want to keep the values that are larger, its either true or false
+    Ok(BooleanArray::new(values, None))
+}
+
+/// Return array where the largest value at each index is kept
+fn keep_larger(lhs: ArrayRef, rhs: ArrayRef) -> Result<ArrayRef> {
+    // True for values that we should keep from the left array
+    let keep_lhs = get_larger(lhs.as_ref(), rhs.as_ref())?;
+
+    let larger = zip(&keep_lhs, &lhs, &rhs)?;
+
+    Ok(larger)
+}
+
+fn keep_larger_scalar<'a>(
+    lhs: &'a ScalarValue,
+    rhs: &'a ScalarValue,
+) -> Result<&'a ScalarValue> {
+    if !lhs.data_type().is_nested() {
+        return if lhs >= rhs { Ok(lhs) } else { Ok(rhs) };
+    }
+
+    // If complex type we can't compare directly as we want null values to be smaller
+    let cmp = make_comparator(
+        lhs.to_array()?.as_ref(),
+        rhs.to_array()?.as_ref(),
+        SORT_OPTIONS,
+    )?;
+
+    if cmp(0, 0).is_ge() {
+        Ok(lhs)
+    } else {
+        Ok(rhs)
+    }
+}
+
+fn find_coerced_type(data_types: &[DataType]) -> Result<DataType> {
+    if data_types.is_empty() {
+        plan_err!("greatest was called without any arguments. It requires at least 1.")
+    } else if let Some(coerced_type) = type_union_resolution(data_types) {
+        Ok(coerced_type)
+    } else {
+        plan_err!("Cannot find a common type for arguments")
+    }
+}
+
+impl ScalarUDFImpl for GreatestFunc {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn name(&self) -> &str {
+        "greatest"
+    }
+
+    fn signature(&self) -> &Signature {
+        &self.signature
+    }
+
+    fn return_type(&self, arg_types: &[DataType]) -> Result<DataType> {
+        Ok(arg_types[0].clone())
+    }
+
+    fn invoke(&self, args: &[ColumnarValue]) -> Result<ColumnarValue> {
+        if args.is_empty() {
+            return exec_err!(
+                "greatest was called with no arguments. It requires at least 1."
+            );
+        }
+
+        // Some engines (e.g. SQL Server) allow greatest with single arg, it's a noop
+        if args.len() == 1 {
+            return Ok(args[0].clone());
+        }
+
+        // Split to scalars and arrays for later optimization
+        let (scalars, arrays): (Vec<_>, Vec<_>) = args.iter().partition(|x| match x {
+            ColumnarValue::Scalar(_) => true,
+            ColumnarValue::Array(_) => false,
+        });
+
+        let mut arrays_iter = arrays.iter().map(|x| match x {
+            ColumnarValue::Array(a) => a,
+            _ => unreachable!(),
+        });
+
+        let first_array = arrays_iter.next();
+
+        let mut largest: ArrayRef;
+
+        // Optimization: merge all scalars into one to avoid recomputing
+        if !scalars.is_empty() {
+            let mut scalars_iter = scalars.iter().map(|x| match x {
+                ColumnarValue::Scalar(s) => s,
+                _ => unreachable!(),
+            });
+
+            // We have at least one scalar
+            let mut largest_scalar = scalars_iter.next().unwrap();
+
+            for scalar in scalars_iter {
+                largest_scalar = keep_larger_scalar(largest_scalar, scalar)?;
+            }
+
+            // If we only have scalars, return the largest one
+            if arrays.is_empty() {
+                return Ok(ColumnarValue::Scalar(largest_scalar.clone()));
+            }
+
+            // We have at least one array
+            let first_array = first_array.unwrap();
+
+            // Start with the largest value
+            largest = keep_larger(
+                Arc::clone(first_array),
+                largest_scalar.to_array_of_size(first_array.len())?,
+            )?;
+        } else {
+            // If we only have arrays, start with the first array
+            // (We must have at least one array)
+            largest = Arc::clone(first_array.unwrap());
+        }
+
+        for array in arrays_iter {
+            largest = keep_larger(Arc::clone(array), largest)?;
+        }
+
+        Ok(ColumnarValue::Array(largest))
+    }
+
+    fn coerce_types(&self, arg_types: &[DataType]) -> Result<Vec<DataType>> {
+        let coerced_type = find_coerced_type(arg_types)?;
+
+        Ok(vec![coerced_type; arg_types.len()])
+    }
+
+    fn documentation(&self) -> Option<&Documentation> {
+        Some(get_greatest_doc())
+    }
+}
+static DOCUMENTATION: OnceLock<Documentation> = OnceLock::new();
+
+fn get_greatest_doc() -> &'static Documentation {
+    DOCUMENTATION.get_or_init(|| {
+        Documentation::builder()
+            .with_doc_section(DOC_SECTION_CONDITIONAL)
+            .with_description("Returns the greatest value in a list of expressions. Returns _null_ if all expressions are _null_.")
+            .with_syntax_example("greatest(expression1[, ..., expression_n])")
+            .with_sql_example(r#"```sql
+> select greatest(4, 7, 5);
++---------------------------+
+| greatest(4,7,5)           |
++---------------------------+
+| 7                         |
++---------------------------+
+```"#,
+            )
+            .with_argument(
+                "expression1, expression_n",
+                "Expressions to compare and return the greatest value.. 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 crate::core;
+    use arrow::datatypes::DataType;
+    use datafusion_expr::ScalarUDFImpl;
+
+    #[test]
+    fn test_greatest_return_types_without_common_supertype_in_arg_type() {
+        let greatest = core::greatest::GreatestFunc::new();
+        let return_type = greatest
+            .coerce_types(&[DataType::Decimal128(10, 3), DataType::Decimal128(10, 4)])
+            .unwrap();
+        assert_eq!(
+            return_type,
+            vec![DataType::Decimal128(11, 4), DataType::Decimal128(11, 4)]
+        );
+    }
+}

datafusion/functions/src/core/mod.rs

@@ -25,6 +25,7 @@ pub mod arrowtypeof;
 pub mod coalesce;
 pub mod expr_ext;
 pub mod getfield;
+pub mod greatest;
 pub mod named_struct;
 pub mod nullif;
 pub mod nvl;
@@ -43,6 +44,7 @@ make_udf_function!(r#struct::StructFunc, STRUCT, r#struct);
 make_udf_function!(named_struct::NamedStructFunc, NAMED_STRUCT, named_struct);
 make_udf_function!(getfield::GetFieldFunc, GET_FIELD, get_field);
 make_udf_function!(coalesce::CoalesceFunc, COALESCE, coalesce);
+make_udf_function!(greatest::GreatestFunc, GREATEST, greatest);
 make_udf_function!(version::VersionFunc, VERSION, version);
 
 pub mod expr_fn {
@@ -80,6 +82,10 @@ pub mod expr_fn {
         coalesce,
         "Returns `coalesce(args...)`, which evaluates to the value of the first expr which is not NULL",
         args,
+    ),(
+        greatest,
+        "Returns `greatest(args...)`, which evaluates to the greatest value in the list of expressions or NULL if all the expressions are NULL",
+        args,
     ));
 
     #[doc = "Returns the value of the field with the given name from the struct"]
@@ -106,6 +112,7 @@ pub fn functions() -> Vec<Arc<ScalarUDF>> {
         // calls to `get_field`
         get_field(),
         coalesce(),
+        greatest(),
         version(),
         r#struct(),
     ]