Fix cast to decimal overflow bug (#3922) (#4081)

close #3920

dbms/src/Functions/FunctionsTiDBConversion.h

@@ -790,21 +790,27 @@ struct TiDBConvertToDecimal
     using FromFieldType = typename FromDataType::FieldType;
 
     template <typename T, typename U>
-    static U toTiDBDecimalInternal(T value, PrecType prec, ScaleType scale, const Context & context)
+    static U toTiDBDecimalInternal(T int_value, PrecType prec, ScaleType scale, const Context & context)
     {
+        // int_value is the value that exposes to user. Such as cast(val to decimal), val is the int_value which used by user.
+        // And val * scale_mul is the scaled_value, which is stored in ColumnDecimal internally.
+        static_assert(std::is_integral_v<T>);
         using UType = typename U::NativeType;
-        auto maxValue = DecimalMaxValue::Get(prec);
-        if (value > maxValue || value < -maxValue)
+        UType scale_mul = getScaleMultiplier<U>(scale);
+
+        Int256 scaled_value = static_cast<Int256>(int_value) * static_cast<Int256>(scale_mul);
+        Int256 scaled_max_value = DecimalMaxValue::Get(prec);
+
+        if (scaled_value > scaled_max_value || scaled_value < -scaled_max_value)
         {
             context.getDAGContext()->handleOverflowError("cast to decimal", Errors::Types::Truncated);
-            if (value > 0)
-                return static_cast<UType>(maxValue);
+            if (int_value > 0)
+                return static_cast<UType>(scaled_max_value);
             else
-                return static_cast<UType>(-maxValue);
+                return static_cast<UType>(-scaled_max_value);
         }
-        UType scale_mul = getScaleMultiplier<U>(scale);
-        U result = static_cast<UType>(value) * scale_mul;
-        return result;
+
+        return static_cast<UType>(scaled_value);
     }
 
     template <typename U>

tests/fullstack-test/expr/cast_as_decimal.test

@@ -0,0 +1,17 @@
+mysql> drop table if exists test.t1;
+mysql> create table test.t1(c1 int);
+mysql> insert into test.t1 values(9999), (-9999), (99), (-99);
+mysql> alter table test.t1 set tiflash replica 1;
+func> wait_table test t1
+mysql> set @@tidb_isolation_read_engines='tiflash'; select cast(c1 as decimal(4, 1)) from test.t1 order by 1;
+cast(c1 as decimal(4, 1))
+-999.9
+-99.0
+99.0
+999.9
+mysql> set @@tidb_isolation_read_engines='tiflash'; select cast(c1 as decimal(2, 2)) from test.t1 order by 1;
+cast(c1 as decimal(2, 2))
+-0.99
+-0.99
+ 0.99
+ 0.99