expression: fix a bug that DML using caseWhen may cause schema change (…

…#20095)

Signed-off-by: wjhuang2016 <huangwenjun1997@gmail.com>

expression/constant_fold.go

@@ -97,7 +97,7 @@ func caseWhenHandler(expr *ScalarFunction) (Expression, bool) {
  foldedExpr.GetType().Decimal = expr.GetType().Decimal
  return foldedExpr, isDeferredConst
  }
- return BuildCastFunction(expr.GetCtx(), foldedExpr, foldedExpr.GetType()), isDeferredConst
+ return foldedExpr, isDeferredConst
  }
  } else {
  // for no-const, here should return directly, because the following branches are unknown to be run or not

expression/integration_test.go

@@ -3592,6 +3592,7 @@ func (s *testIntegrationSuite) TestAggregationBuiltin(c *C) {
  defer s.cleanEnv(c)
  tk := testkit.NewTestKit(c, s.store)
  tk.MustExec("use test")
+ tk.MustExec("drop table if exists t")
  tk.MustExec("create table t(a decimal(7, 6))")
  tk.MustExec("insert into t values(1.123456), (1.123456)")
  result := tk.MustQuery("select avg(a) from t")
@@ -3618,6 +3619,19 @@ func (s *testIntegrationSuite) TestAggregationBuiltin(c *C) {
  result.Check(testkit.Rows("18446744073709551615"))
 }
 
+func (s *testIntegrationSuite) Test19387(c *C) {
+ tk := testkit.NewTestKit(c, s.store)
+ tk.MustExec("USE test;")
+
+ tk.MustExec("drop table if exists t;")
+ tk.MustExec("create table t(a decimal(16, 2));")
+ tk.MustExec("select sum(case when 1 then a end) from t group by a;")
+ res := tk.MustQuery("show create table t")
+ c.Assert(len(res.Rows()), Equals, 1)
+ str := res.Rows()[0][1].(string)
+ c.Assert(strings.Contains(str, "decimal(16,2)"), IsTrue)
+}
+
 func (s *testIntegrationSuite) TestAggregationBuiltinBitOr(c *C) {
  defer s.cleanEnv(c)
  tk := testkit.NewTestKit(c, s.store)