expression, json: fix cast json as string with shorter length

expression/builtin_cast.go

@@ -1722,7 +1722,11 @@ func (b *builtinCastJSONAsStringSig) evalString(row chunk.Row) (res string, isNu
 	if isNull || err != nil {
 		return res, isNull, err
 	}
-	return val.String(), false, nil
+	s, err := types.ProduceStrWithSpecifiedTp(val.String(), b.tp, b.ctx.GetSessionVars().StmtCtx, false)
+	if err != nil {
+		return res, false, err
+	}
+	return s, false, nil
 }
 
 type builtinCastJSONAsTimeSig struct {

expression/builtin_cast_test.go

@@ -719,6 +719,13 @@ func TestCastFuncSig(t *testing.T) {
 			3,
 			chunk.MutRowFromDatums([]types.Datum{types.NewStringDatum("你好world")}),
 		},
+		// cast json as string
+		{
+			&Column{RetType: types.NewFieldType(mysql.TypeJSON), Index: 0},
+			fmt.Sprintf(`"%s`, curTimeString[:2]),
+			3,
+			chunk.MutRowFromDatums([]types.Datum{jsonTime}),
+		},
 	}
 	for i, c := range castToStringCases2 {
 		args := []Expression{c.before}
@@ -741,6 +748,8 @@ func TestCastFuncSig(t *testing.T) {
 		case 5:
 			stringFunc.tp.SetCharset(charset.CharsetUTF8)
 			sig = &builtinCastStringAsStringSig{stringFunc}
+		case 6:
+			sig = &builtinCastJSONAsStringSig{stringFunc}
 		}
 		res, isNull, err := sig.evalString(c.row.ToRow())
 		require.False(t, isNull)

expression/builtin_cast_vec.go

@@ -1186,7 +1186,11 @@ func (b *builtinCastJSONAsStringSig) vecEvalString(input *chunk.Chunk, result *c
 			result.AppendNull()
 			continue
 		}
-		result.AppendString(buf.GetJSON(i).String())
+		s, err := types.ProduceStrWithSpecifiedTp(buf.GetJSON(i).String(), b.tp, b.ctx.GetSessionVars().StmtCtx, false)
+		if err != nil {
+			return err
+		}
+		result.AppendString(s)
 	}
 	return nil
 }