evalengine: Implement FROM_DAYS

go/vt/vtgate/evalengine/compiler_asm.go

@@ -3795,6 +3795,23 @@ func (asm *assembler) Fn_LAST_DAY() {
 	}, "FN LAST_DAY DATETIME(SP-1)")
 }
 
+func (asm *assembler) Fn_FROM_DAYS() {
+	asm.emit(func(env *ExpressionEnv) int {
+		arg := env.vm.stack[env.vm.sp-1].(*evalInt64)
+		if arg == nil {
+			return 1
+		}
+
+		d := fromDays(env.currentTimezone(), arg.i)
+		if d == nil || d.IsZero() {
+			env.vm.stack[env.vm.sp-1] = nil
+		} else {
+			env.vm.stack[env.vm.sp-1] = env.vm.arena.newEvalDate(*d)
+		}
+		return 1
+	}, "FN FROM_DAYS INT64(SP-1)")
+}
+
 func (asm *assembler) Fn_QUARTER() {
 	asm.emit(func(env *ExpressionEnv) int {
 		if env.vm.stack[env.vm.sp-1] == nil {

go/vt/vtgate/evalengine/fn_time.go

@@ -111,6 +111,10 @@ type (
 		CallExpr
 	}
 
+	builtinFromDays struct {
+		CallExpr
+	}
+
 	builtinQuarter struct {
 		CallExpr
 	}
@@ -173,6 +177,7 @@ var _ IR = (*builtinMinute)(nil)
 var _ IR = (*builtinMonth)(nil)
 var _ IR = (*builtinMonthName)(nil)
 var _ IR = (*builtinLastDay)(nil)
+var _ IR = (*builtinFromDays)(nil)
 var _ IR = (*builtinQuarter)(nil)
 var _ IR = (*builtinSecond)(nil)
 var _ IR = (*builtinTime)(nil)
@@ -1249,6 +1254,53 @@ func (call *builtinLastDay) compile(c *compiler) (ctype, error) {
 	return ctype{Type: sqltypes.Date, Flag: arg.Flag | flagNullable}, nil
 }
 
+func fromDays(loc *time.Location, d int64) *datetime.Date {
+	// 3652424 days corresponds to maximum date i.e. 9999-12-31
+	// mysql returns 0000-00-00 for days below 366
+	if d > 3652424 || d < 366 {
+		return nil
+	}
+
+	t := time.Date(1, time.January, 1, 0, 0, 0, 0, loc).AddDate(0, 0, int(d-366))
+	dt := datetime.NewDateFromStd(t)
+	return &dt
+}
+
+func (b *builtinFromDays) eval(env *ExpressionEnv) (eval, error) {
+	d, err := b.arg1(env)
+	if err != nil {
+		return nil, err
+	}
+	if d == nil {
+		return nil, nil
+	}
+	days := evalToInt64(d).i
+	dt := fromDays(env.currentTimezone(), days)
+
+	if dt == nil {
+		return nil, nil
+	}
+	return newEvalDate(*dt, env.sqlmode.AllowZeroDate()), nil
+}
+
+func (call *builtinFromDays) compile(c *compiler) (ctype, error) {
+	arg, err := call.Arguments[0].compile(c)
+	if err != nil {
+		return ctype{}, err
+	}
+
+	skip := c.compileNullCheck1(arg)
+	switch arg.Type {
+	case sqltypes.Int64:
+	default:
+		c.asm.Convert_xi(1)
+	}
+
+	c.asm.Fn_FROM_DAYS()
+	c.asm.jumpDestination(skip)
+	return ctype{Type: sqltypes.Date, Flag: arg.Flag | flagNullable}, nil
+}
+
 func (b *builtinQuarter) eval(env *ExpressionEnv) (eval, error) {
 	date, err := b.arg1(env)
 	if err != nil {

go/vt/vtgate/evalengine/testcases/cases.go

@@ -131,6 +131,7 @@ var Cases = []TestCase{
 	{Run: FnMonth},
 	{Run: FnMonthName},
 	{Run: FnLastDay},
+	{Run: FnFromDays},
 	{Run: FnQuarter},
 	{Run: FnSecond},
 	{Run: FnTime},
@@ -1766,6 +1767,25 @@ func FnLastDay(yield Query) {
 	}
 }
 
+func FnFromDays(yield Query) {
+	for _, d := range inputConversions {
+		yield(fmt.Sprintf("FROM_DAYS(%s)", d), nil)
+	}
+
+	days := []string{
+		"0",
+		"1",
+		"366",
+		"365242",
+		"3652424",
+		"3652425",
+	}
+
+	for _, d := range days {
+		yield(fmt.Sprintf("FROM_DAYS(%s)", d), nil)
+	}
+}
+
 func FnQuarter(yield Query) {
 	for _, d := range inputConversions {
 		yield(fmt.Sprintf("QUARTER(%s)", d), nil)

go/vt/vtgate/evalengine/translate_builtin.go

@@ -419,6 +419,11 @@ func (ast *astCompiler) translateFuncExpr(fn *sqlparser.FuncExpr) (IR, error) {
 			return nil, argError(method)
 		}
 		return &builtinLastDay{CallExpr: call}, nil
+	case "from_days":
+		if len(args) != 1 {
+			return nil, argError(method)
+		}
+		return &builtinFromDays{CallExpr: call}, nil
 	case "quarter":
 		if len(args) != 1 {
 			return nil, argError(method)