[fix][libexpr] Fix decimal neg operation issue.

Author: nokiaMS

src/types/decimal/decimal.cc

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 #include <iostream>
+#include <sstream>
 #include "decimal.h"
 
 namespace dingodb {
@@ -382,9 +383,9 @@ Decimal Decimal::operator-() const {
     result.append(str, curPos);
   }
   else {
-    result.append(str, curPos, exp);
-    result.push_back('.');
-    result.append(str, curPos + exp);
+    std::ostringstream oss;
+    oss << 0 - v;
+    result = oss.str();
   }
 
   return std::move(Decimal(result));

test/types/test_type_decimal.cc

@@ -360,6 +360,10 @@ TEST(TestTypeDecimal, DecimalTest) {
   //Test dec > dec.
   ASSERT_FALSE(Decimal(std::string("123.123")) > Decimal(std::string("123.124")));
 
+  //Test -dec.
+  //This case will append '.00' in serial stage.
+  ASSERT_EQ((-Decimal(std::string("10.00"))).toString(), std::string("-10"));
+
   //Test -dec.
   ASSERT_EQ((-Decimal(std::string("123.123"))).toString(), std::string("-123.123"));