bkietz commented on a change in pull request #10364:
URL: https://github.com/apache/arrow/pull/10364#discussion_r646711505
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File path: cpp/src/arrow/compute/kernels/scalar_arithmetic.cc
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@@ -428,12 +524,69 @@ ArrayKernelExec ArithmeticExecFromOp(detail::GetTypeId
get_id) {
}
}
+// calculate output precision/scale and args rescaling per operation type
+Result<std::shared_ptr<DataType>> GetDecimalBinaryOutput(
+ const std::string& op, const std::vector<ValueDescr>& values,
+ std::vector<std::shared_ptr<DataType>>* replaced = nullptr) {
+ const auto& left_type = checked_pointer_cast<DecimalType>(values[0].type);
+ const auto& right_type = checked_pointer_cast<DecimalType>(values[1].type);
+
+ const int32_t p1 = left_type->precision(), s1 = left_type->scale();
+ const int32_t p2 = right_type->precision(), s2 = right_type->scale();
+ if (s1 < 0 || s2 < 0) {
+ return Status::NotImplemented("Decimals with negative scales not
supported");
+ }
+
+ int32_t out_prec, out_scale;
+ int32_t left_scaleup = 0, right_scaleup = 0;
+
+ // decimal upscaling behaviour references amazon redshift
+ //
https://docs.aws.amazon.com/redshift/latest/dg/r_numeric_computations201.html
+ if (op.find("add") == 0 || op.find("subtract") == 0) {
+ out_scale = std::max(s1, s2);
+ out_prec = std::max(p1 - s1, p2 - s2) + 1 + out_scale;
+ left_scaleup = out_scale - s1;
+ right_scaleup = out_scale - s2;
+ } else if (op.find("multiply") == 0) {
+ out_scale = s1 + s2;
+ out_prec = p1 + p2 + 1;
+ } else if (op.find("divide") == 0) {
+ out_scale = std::max(4, s1 + p2 - s2 + 1);
+ out_prec = p1 - s1 + s2 + out_scale; // >= p1 + p2 + 1
+ left_scaleup = out_prec - p1;
+ } else {
+ return Status::Invalid("Invalid decimal operation: ", op);
+ }
+
+ const auto id = left_type->id();
Review comment:
If you want to defer mixed width to a follow up that's acceptable, but
DispatchDecimal should fail gracefully for that case
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