enhancement: Avoid calling reflect.New() when passing in slice of values to Scan() (#5388)

* fix: reduce allocations when slice of values

* chore[test]: Add benchmark for scan

* chore[test]: add bench for scan slice

* chore[test]: add bench for slice pointer and improve tests

* chore[test]: make sure database is empty when doing slice tests

* fix[test]: correct sql delete statement

* enhancement: skip new if rows affected = 0

scan.go

@@ -237,6 +237,7 @@ func Scan(rows Rows, db *DB, mode ScanMode) {
 		switch reflectValue.Kind() {
 		case reflect.Slice, reflect.Array:
 			var elem reflect.Value
+			recyclableStruct := reflect.New(reflectValueType)
 
 			if !update || reflectValue.Len() == 0 {
 				update = false
@@ -261,7 +262,11 @@ func Scan(rows Rows, db *DB, mode ScanMode) {
 						}
 					}
 				} else {
+					if isPtr && db.RowsAffected > 0 {
 						elem = reflect.New(reflectValueType)
+					} else {
+						elem = recyclableStruct
+					}
 				}
 
 				db.scanIntoStruct(rows, elem, values, fields, joinFields)

tests/benchmark_test.go

@@ -1,6 +1,7 @@
 package tests_test
 
 import (
+	"fmt"
 	"testing"
 
 	. "gorm.io/gorm/utils/tests"
@@ -24,6 +25,45 @@ func BenchmarkFind(b *testing.B) {
 	}
 }
 
+func BenchmarkScan(b *testing.B) {
+	user := *GetUser("scan", Config{})
+	DB.Create(&user)
+
+	var u User
+	b.ResetTimer()
+	for x := 0; x < b.N; x++ {
+		DB.Raw("select * from users where id = ?", user.ID).Scan(&u)
+	}
+}
+
+func BenchmarkScanSlice(b *testing.B) {
+	DB.Exec("delete from users")
+	for i := 0; i < 10_000; i++ {
+		user := *GetUser(fmt.Sprintf("scan-%d", i), Config{})
+		DB.Create(&user)
+	}
+
+	var u []User
+	b.ResetTimer()
+	for x := 0; x < b.N; x++ {
+		DB.Raw("select * from users").Scan(&u)
+	}
+}
+
+func BenchmarkScanSlicePointer(b *testing.B) {
+	DB.Exec("delete from users")
+	for i := 0; i < 10_000; i++ {
+		user := *GetUser(fmt.Sprintf("scan-%d", i), Config{})
+		DB.Create(&user)
+	}
+
+	var u []*User
+	b.ResetTimer()
+	for x := 0; x < b.N; x++ {
+		DB.Raw("select * from users").Scan(&u)
+	}
+}
+
 func BenchmarkUpdate(b *testing.B) {
 	user := *GetUser("find", Config{})
 	DB.Create(&user)