[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #2241: fix: find the right wider decimal datatype for comparison operation

[GitBox <gi...@apache.org>]

alamb commented on code in PR #2241:
URL: https://github.com/apache/arrow-datafusion/pull/2241#discussion_r851340950


##########
datafusion/physical-expr/src/coercion_rule/binary_rule.rs:
##########
@@ -178,15 +178,30 @@ fn get_comparison_common_decimal_type(
         }
     };
     match (decimal_type, &other_decimal_type) {
-        (DataType::Decimal(p1, s1), DataType::Decimal(p2, s2)) => {
-            let new_precision = p1.max(p2);
-            let new_scale = s1.max(s2);
-            Some(DataType::Decimal(*new_precision, *new_scale))
+        (d1 @ DataType::Decimal(_, _), d2 @ DataType::Decimal(_, _)) => {
+            get_wider_decimal_type(d1, d2)
         }
         _ => None,
     }
 }
 
+// Find the winder decimal type when both types are decimal.

Review Comment:
   ```suggestion
   // Returns a `DataType::Decimal` that can store any value from either 
   // `lhs_decimal_type` and `rhs_decimal_type`
   ```



##########
datafusion/physical-expr/src/coercion_rule/binary_rule.rs:
##########
@@ -178,15 +178,30 @@ fn get_comparison_common_decimal_type(
         }
     };
     match (decimal_type, &other_decimal_type) {
-        (DataType::Decimal(p1, s1), DataType::Decimal(p2, s2)) => {
-            let new_precision = p1.max(p2);
-            let new_scale = s1.max(s2);
-            Some(DataType::Decimal(*new_precision, *new_scale))
+        (d1 @ DataType::Decimal(_, _), d2 @ DataType::Decimal(_, _)) => {
+            get_wider_decimal_type(d1, d2)
         }
         _ => None,
     }
 }
 
+// Find the winder decimal type when both types are decimal.
+// The result decimal type is (max(s1, s2) + max(p1-s1, p2-s2), max(s1, s2)).
+fn get_wider_decimal_type(
+    lhs_decimal_type: &DataType,
+    rhs_type: &DataType,
+) -> Option<DataType> {
+    match (lhs_decimal_type, rhs_type) {
+        (DataType::Decimal(p1, s1), DataType::Decimal(p2, s2)) => {
+            // max(s1, s2) + max(p1-s1, p2-s2), max(s1, s2)
+            let s = *s1.max(s2);
+            let range = (p1 - s1).max(p2 - s2);

Review Comment:
   👍  I see that the precision includes the scale 👍 



##########
datafusion/physical-expr/src/coercion_rule/binary_rule.rs:
##########
@@ -557,15 +571,17 @@ mod tests {
             DataType::Float32,
             DataType::Float64,
             DataType::Decimal(38, 10),
+            DataType::Decimal(20, 8),
         ];
         let result_types = [
             DataType::Decimal(20, 3),
             DataType::Decimal(20, 3),
             DataType::Decimal(20, 3),
-            DataType::Decimal(20, 3),
-            DataType::Decimal(20, 7),
-            DataType::Decimal(30, 15),
+            DataType::Decimal(23, 3),
+            DataType::Decimal(24, 7),
+            DataType::Decimal(32, 15),
             DataType::Decimal(38, 10),
+            DataType::Decimal(25, 8),

Review Comment:
   👍 



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