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

[Bexanderthebex]

What did this pull request do?

Tries to optimize Scan() method by recycling a struct when a user passes a slice of values to prevent calling reflect.New() every time regardless of the type.

User Case Description

When passing a slice of values, the run time is the same as passing in a slice of pointers. Users should not be penalized for such.

[Bexanderthebex]

Relevant: #5372

[umitanuki]

Suggested change

	var u User
	b.ResetTimer()
	var u User

[umitanuki]

Suggested change

	var u []User
	b.ResetTimer()
	var u []User

[umitanuki]

Suggested change

	func BenchmarkScanSlicePointer(b *testing.B) {
	for i := 0; i < 10_000; i++ {
	user := *GetUser(fmt.Sprintf("scan-%d", i), Config{})
	DB.Create(&user)
	}
	b.ResetTimer()
	var u []*User
	for x := 0; x < b.N; x++ {
	DB.Raw("select * from users").Scan(&u)
	}
	}

[umitanuki]

My benchmark result suggests something promising.

BenchmarkScanSlice-12           	   9	 188888672 ns/op	66993973 B/op	 1011373 allocs/op
BenchmarkScanSlicePointer-12    	   3	 355558214 ns/op	61715941 B/op	 2051542 allocs/op

[Bexanderthebex]

@umitanuki Agreed, benchmark according to my benchmark, improvement is ~50%

BenchmarkScanSlice-12                  8         213268227 ns/op        68680647 B/op    1092749 allocs/op
BenchmarkScanSlicePointer-12      4         370261906 ns/op        79490372 B/op    2644559 allocs/op

[Bexanderthebex]

New:

BenchmarkScanSlice-12             14          73788941 ns/op        31725140 B/op     499899 allocs/op
BenchmarkScanSlicePointer-12 15          69792858 ns/op        15323992 B/op     509812 allocs/op

Old:

BenchmarkScanSlice-12               13          83697560 ns/op        36216732 B/op     509938 allocs/op
BenchmarkScanSlicePointer-12   14          74349205 ns/op        15321940 B/op     509806 allocs/op

cc: @a631807682

[Bexanderthebex]

@jinzhu when you have time, can you take a look? thank you!

[jinzhu]

We can change this to if isPtr && db.RowsAffected > 0?

Seems not necessary to create a new struct for the first record.

[Bexanderthebex]

@jinzhu do you mean if isPtr && db.RowsAffected == 0?

[Bexanderthebex]

Oh no, you are correct, making this change now.

[jinzhu]

@jinzhu when you have time, can you take a look? thank you!

Just submitted a small review.

In general looks great, thank you for your contribution. ❤️