fix decimal divide problem

Co-authored-by ZhangHuiGui <2689496754@qq.com>
Co-authored-by laotan332 <qinpengxiang@outlook.com>

Author: ZhangHuiGui

cpp/src/arrow/compute/expression_test.cc

@@ -944,6 +944,41 @@ TEST(Expression, ExecuteDictionaryTransparent) {
   ])"));
 }
 
+TEST(Expression, ExecuteDecimalDivide) {
+  // GH-40911 Decimal divide promotion rule output wrong precision and scale
+  // in expression Bind when scale1 >= scale2
+  //
+  // scale1 < scale2
+  ExpectExecute(
+      call("divide", {field_ref("a"), field_ref("b")}),
+      ArrayFromJSON(struct_({field("a", decimal128(6, 1)), field("b", decimal128(7, 2))}),
+                    R"([
+        {"a": "0.7", "b": "-2.17"},
+        {"a": "2.9", "b": "9.11"},
+        {"a": "3.1", "b" : "12.31"}
+      ])"));
+
+  // scale1 == scale2
+  ExpectExecute(
+      call("divide", {field_ref("a"), field_ref("b")}),
+      ArrayFromJSON(struct_({field("a", decimal128(6, 1)), field("b", decimal128(6, 1))}),
+                    R"([
+        {"a": "0.7", "b": "-2.1"},
+        {"a": "2.9", "b": "9.1"},
+        {"a": "3.1", "b" : "12.3"}
+      ])"));
+
+  // scale1 > scale2
+  ExpectExecute(
+      call("divide", {field_ref("a"), field_ref("b")}),
+      ArrayFromJSON(struct_({field("a", decimal128(6, 2)), field("b", decimal128(6, 1))}),
+                    R"([
+        {"a": "0.71", "b": "-2.1"},
+        {"a": "2.92", "b": "9.1"},
+        {"a": "3.17", "b" : "12.3"}
+      ])"));
+}
+
 void ExpectIdenticalIfUnchanged(Expression modified, Expression original) {
   if (modified == original) {
     // no change -> must be identical

cpp/src/arrow/compute/kernels/codegen_internal.cc

@@ -428,13 +428,9 @@ Status CastBinaryDecimalArgs(DecimalPromotion promotion, std::vector<TypeHolder>
       right_scaleup = std::max(s1, s2) - s2;
       break;
     }
-    case DecimalPromotion::kMultiply: {
-      left_scaleup = right_scaleup = 0;
-      break;
-    }
+    case DecimalPromotion::kMultiply:
     case DecimalPromotion::kDivide: {
-      left_scaleup = std::max(4, s1 + p2 - s2 + 1) + s2 - s1;
-      right_scaleup = 0;
+      left_scaleup = right_scaleup = 0;
       break;
     }
     default:

cpp/src/arrow/compute/kernels/scalar_arithmetic.cc

@@ -541,13 +541,8 @@ Result<TypeHolder> ResolveDecimalDivisionOutput(KernelContext*,
       types,
       [](int32_t p1, int32_t s1, int32_t p2,
          int32_t s2) -> Result<std::pair<int32_t, int32_t>> {
-        if (s1 < s2) {
-          return Status::Invalid("Division of two decimal types scale1 < scale2. ", "(",
-                                 s1, s2, ").");
-        }
-        DCHECK_GE(s1, s2);
-        const int32_t scale = s1 - s2;
-        const int32_t precision = p1;
+        const int32_t scale = std::max(4, s1 + p2 - s2 + 1);
+        const int32_t precision = p1 - s1 + s2 + scale;
         return std::make_pair(precision, scale);
       });
 }