AdamGS commented on code in PR #17808:
URL: https://github.com/apache/datafusion/pull/17808#discussion_r2387700919


##########
datafusion/expr-common/src/type_coercion/binary.rs:
##########
@@ -955,28 +963,106 @@ pub fn decimal_coercion(lhs_type: &DataType, rhs_type: 
&DataType) -> Option<Data
 
     match (lhs_type, rhs_type) {
         // Prefer decimal data type over floating point for comparison 
operation
+        (Decimal32(_, _), Decimal32(_, _)) => get_wider_decimal_type(lhs_type, 
rhs_type),
+        (Decimal32(_, _), Decimal64(_, _) | Decimal128(_, _) | Decimal256(_, 
_)) => {
+            get_wider_decimal_type_cross_variant(lhs_type, rhs_type)
+        }
+        (Decimal32(_, _), _) => get_common_decimal_type(lhs_type, rhs_type),
+        (Decimal64(_, _), Decimal64(_, _)) => get_wider_decimal_type(lhs_type, 
rhs_type),
+        (Decimal64(_, _), Decimal32(_, _) | Decimal128(_, _) | Decimal256(_, 
_)) => {
+            get_wider_decimal_type_cross_variant(lhs_type, rhs_type)
+        }
+        (Decimal64(_, _), _) => get_common_decimal_type(lhs_type, rhs_type),
         (Decimal128(_, _), Decimal128(_, _)) => {
             get_wider_decimal_type(lhs_type, rhs_type)
         }
+        (Decimal128(_, _), Decimal32(_, _) | Decimal64(_, _) | Decimal256(_, 
_)) => {
+            get_wider_decimal_type_cross_variant(lhs_type, rhs_type)
+        }
         (Decimal128(_, _), _) => get_common_decimal_type(lhs_type, rhs_type),
-        (_, Decimal128(_, _)) => get_common_decimal_type(rhs_type, lhs_type),
         (Decimal256(_, _), Decimal256(_, _)) => {
             get_wider_decimal_type(lhs_type, rhs_type)
         }
+        (Decimal256(_, _), Decimal32(_, _) | Decimal64(_, _) | Decimal128(_, 
_)) => {
+            get_wider_decimal_type_cross_variant(lhs_type, rhs_type)
+        }
         (Decimal256(_, _), _) => get_common_decimal_type(lhs_type, rhs_type),
+        (_, Decimal32(_, _)) => get_common_decimal_type(rhs_type, lhs_type),
+        (_, Decimal64(_, _)) => get_common_decimal_type(rhs_type, lhs_type),
+        (_, Decimal128(_, _)) => get_common_decimal_type(rhs_type, lhs_type),
         (_, Decimal256(_, _)) => get_common_decimal_type(rhs_type, lhs_type),
         (_, _) => None,
     }
 }
 
+/// Handle cross-variant decimal widening by choosing the larger variant
+fn get_wider_decimal_type_cross_variant(
+    lhs_type: &DataType,
+    rhs_type: &DataType,
+) -> Option<DataType> {
+    use arrow::datatypes::DataType::*;
+
+    let (p1, s1) = match lhs_type {
+        Decimal32(p, s) => (*p, *s),
+        Decimal64(p, s) => (*p, *s),
+        Decimal128(p, s) => (*p, *s),
+        Decimal256(p, s) => (*p, *s),
+        _ => return None,
+    };
+
+    let (p2, s2) = match rhs_type {
+        Decimal32(p, s) => (*p, *s),
+        Decimal64(p, s) => (*p, *s),
+        Decimal128(p, s) => (*p, *s),
+        Decimal256(p, s) => (*p, *s),
+        _ => return None,
+    };
+
+    // max(s1, s2) + max(p1-s1, p2-s2), max(s1, s2)
+    let s = s1.max(s2);
+    let range = (p1 as i8 - s1).max(p2 as i8 - s2);
+    let required_precision = (range + s) as u8;

Review Comment:
   Done as part of fd1f04333bcbd8f5381bd7dd031d1eba5f94a334



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