StructMapper: schedule scans by column index (O(C²) → O(C))

bench_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 	"testing"
 	"time"
@@ -18,6 +19,7 @@ import (
 var (
 	db       *sql.DB
 	dataSize = 100
+	wideSize = 1000
 )
 
 func TestMain(m *testing.M) {
@@ -37,6 +39,19 @@ func TestMain(m *testing.M) {
 		panic(err)
 	}
 
+	for _, w := range []struct {
+		table   string
+		numCols int
+	}{
+		{"wide5", 5},
+		{"wide15", 15},
+		{"wide45", 45},
+	} {
+		if err := prepareWideData(ctx, w.table, w.numCols); err != nil {
+			panic(err)
+		}
+	}
+
 	exitVal := m.Run()
 
 	os.Exit(exitVal)
@@ -78,6 +93,28 @@ func BenchmarkScanOne(b *testing.B) {
 	}
 }
 
+func BenchmarkScanWide5(b *testing.B)  { benchmarkScanWide[Wide5](b, "wide5") }
+func BenchmarkScanWide15(b *testing.B) { benchmarkScanWide[Wide15](b, "wide15") }
+func BenchmarkScanWide45(b *testing.B) { benchmarkScanWide[Wide45](b, "wide45") }
+
+func benchmarkScanWide[T any](b *testing.B, table string) {
+	b.StopTimer()
+	ctx := context.Background()
+
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		rows, err := db.Query("SELECT|" + table + "||")
+		if err != nil {
+			panic(err)
+		}
+		b.StartTimer()
+		if _, err := AllFromRows(ctx, StructMapper[T](), rows); err != nil {
+			panic(err)
+		}
+		rows.Close()
+	}
+}
+
 func prepareData(ctx context.Context) error {
 	create := "CREATE|user|id=int64,username=string,password=string"
 	create += ",email=string,mobile_phone=string,company=string,avatar_url=string"
@@ -108,6 +145,107 @@ func prepareData(ctx context.Context) error {
 	return nil
 }
 
+func prepareWideData(ctx context.Context, table string, numCols int) error {
+	colDefs := make([]string, numCols)
+	colAssigns := make([]string, numCols)
+	for i := range colDefs {
+		colDefs[i] = fmt.Sprintf("col_%d=string", i)
+		colAssigns[i] = fmt.Sprintf("col_%d=?", i)
+	}
+
+	create := fmt.Sprintf("CREATE|%s|%s", table, strings.Join(colDefs, ","))
+	if _, err := db.ExecContext(ctx, create); err != nil {
+		return err
+	}
+
+	insert := fmt.Sprintf("INSERT|%s|%s", table, strings.Join(colAssigns, ","))
+	args := make([]any, numCols)
+	for i := 0; i < wideSize; i++ {
+		for c := range args {
+			args[c] = fmt.Sprintf("value_%d_%d", i, c)
+		}
+		if _, err := db.ExecContext(ctx, insert, args...); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+type Wide5 struct {
+	Col0 string `db:"col_0"`
+	Col1 string `db:"col_1"`
+	Col2 string `db:"col_2"`
+	Col3 string `db:"col_3"`
+	Col4 string `db:"col_4"`
+}
+
+type Wide15 struct {
+	Col0  string `db:"col_0"`
+	Col1  string `db:"col_1"`
+	Col2  string `db:"col_2"`
+	Col3  string `db:"col_3"`
+	Col4  string `db:"col_4"`
+	Col5  string `db:"col_5"`
+	Col6  string `db:"col_6"`
+	Col7  string `db:"col_7"`
+	Col8  string `db:"col_8"`
+	Col9  string `db:"col_9"`
+	Col10 string `db:"col_10"`
+	Col11 string `db:"col_11"`
+	Col12 string `db:"col_12"`
+	Col13 string `db:"col_13"`
+	Col14 string `db:"col_14"`
+}
+
+type Wide45 struct {
+	Col0  string `db:"col_0"`
+	Col1  string `db:"col_1"`
+	Col2  string `db:"col_2"`
+	Col3  string `db:"col_3"`
+	Col4  string `db:"col_4"`
+	Col5  string `db:"col_5"`
+	Col6  string `db:"col_6"`
+	Col7  string `db:"col_7"`
+	Col8  string `db:"col_8"`
+	Col9  string `db:"col_9"`
+	Col10 string `db:"col_10"`
+	Col11 string `db:"col_11"`
+	Col12 string `db:"col_12"`
+	Col13 string `db:"col_13"`
+	Col14 string `db:"col_14"`
+	Col15 string `db:"col_15"`
+	Col16 string `db:"col_16"`
+	Col17 string `db:"col_17"`
+	Col18 string `db:"col_18"`
+	Col19 string `db:"col_19"`
+	Col20 string `db:"col_20"`
+	Col21 string `db:"col_21"`
+	Col22 string `db:"col_22"`
+	Col23 string `db:"col_23"`
+	Col24 string `db:"col_24"`
+	Col25 string `db:"col_25"`
+	Col26 string `db:"col_26"`
+	Col27 string `db:"col_27"`
+	Col28 string `db:"col_28"`
+	Col29 string `db:"col_29"`
+	Col30 string `db:"col_30"`
+	Col31 string `db:"col_31"`
+	Col32 string `db:"col_32"`
+	Col33 string `db:"col_33"`
+	Col34 string `db:"col_34"`
+	Col35 string `db:"col_35"`
+	Col36 string `db:"col_36"`
+	Col37 string `db:"col_37"`
+	Col38 string `db:"col_38"`
+	Col39 string `db:"col_39"`
+	Col40 string `db:"col_40"`
+	Col41 string `db:"col_41"`
+	Col42 string `db:"col_42"`
+	Col43 string `db:"col_43"`
+	Col44 string `db:"col_44"`
+}
+
 type Userss struct {
 	ID           int       `db:"id"`
 	UserName     string    `db:"username"`

exec_test.go

@@ -310,6 +310,25 @@ func TestStruct(t *testing.T) {
 	})
 }
 
+func TestStructDuplicateColumns(t *testing.T) {
+	// Duplicate column names in the result set (e.g. `SELECT id, id`)
+	// all scan into the same field.
+	// Previously, every duplicate was scheduled to the first occurrence,
+	// leaving the others without a destination and failing with
+	// "no destination for column id".
+	user1 := User{ID: 1, Name: "foo"}
+	user2 := User{ID: 2, Name: "bar"}
+
+	testQuery(t, "duplicate columns", queryCase[User]{
+		columns:   strstr{{"id", "int64"}, {"name", "string"}},
+		rows:      rows{[]any{1, "foo"}, []any{2, "bar"}},
+		query:     []string{"id", "id", "name"},
+		mapper:    StructMapper[User](),
+		expectOne: user1,
+		expectAll: []User{user1, user2},
+	})
+}
+
 func TestAllowUnknownColumns(t *testing.T) {
 	type testStruct struct {
 		ID  int64

mapper.go

@@ -23,6 +23,7 @@ func (v visited) copy() visited {
 
 type mapinfo struct {
 	name      string
+	colIndex  int
 	position  []int
 	init      [][]int
 	isPointer bool

mapper_struct.go

@@ -196,7 +196,7 @@ func (s regular[T]) regular() (func(*Row) (any, error), func(any) (T, error)) {
 				}
 
 				fv := row.FieldByIndex(info.position)
-				v.ScheduleScanByNameX(info.name, fv.Addr())
+				v.ScheduleScanByIndexX(info.colIndex, fv.Addr())
 			}
 
 			return row, nil
@@ -229,7 +229,7 @@ func (s regular[T]) allOptions() (func(*Row) (any, error), func(any) (T, error))
 					row[i] = reflect.New(ft)
 				}
 
-				v.ScheduleScanByNameX(info.name, row[i])
+				v.ScheduleScanByIndexX(info.colIndex, row[i])
 			}
 
 			return row, nil

mapper_test.go

@@ -255,6 +255,17 @@ func TestStructMapper(t *testing.T) {
 		ExpectedVal: User{ID: 1, Name: "The Name"},
 	})
 
+	// Each occurrence of a duplicated column scans into the same field,
+	// so the value of the last occurrence wins.
+	RunStructMapperTest(t, "duplicate columns", MapperTest[User]{
+		row: &Row{
+			columns: columnNames("id", "id"),
+		},
+		scanned:     []any{1, 2},
+		Mapper:      StructMapper[User](),
+		ExpectedVal: User{ID: 2},
+	})
+
 	RunStructMapperTest(t, "with pointer columns 1", MapperTest[PtrUser1]{
 		row: &Row{
 			columns: columnNames("id", "name", "created_at", "updated_at"),

source.go

@@ -240,7 +240,7 @@ func (s *mapperSourceImpl) setMappings(typ reflect.Type, prefix string, v visite
 func filterColumns(c cols, m mapping, prefix string) (mapping, error) {
 	// Filter the mapping so we only ask for the available columns
 	filtered := make(mapping, 0, len(c))
-	for _, name := range c {
+	for colIdx, name := range c {
 		key := name
 		if prefix != "" {
 			if !strings.HasPrefix(name, prefix) {
@@ -253,6 +253,7 @@ func filterColumns(c cols, m mapping, prefix string) (mapping, error) {
 		for _, info := range m {
 			if key == info.name {
 				info.name = name
+				info.colIndex = colIdx
 				filtered = append(filtered, info)
 				break
 			}