chore: optimize multi-user RSS feed generation by fixing N+1 query (#5749)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

server/router/rss/rss.go

@@ -211,18 +211,24 @@ func (s *RSSService) generateRSSFromMemoList(ctx context.Context, memoList []*st
 		creatorMap[user.ID] = user
 	} else {
 		// Multi-user feed - batch load all unique creators
-		creatorIDs := make(map[int32]bool)
+		creatorIDList := []int32{}
+		creatorIDMap := make(map[int32]bool)
 		for _, memo := range memoList[:itemCountLimit] {
-			creatorIDs[memo.CreatorID] = true
+			if !creatorIDMap[memo.CreatorID] {
+				creatorIDList = append(creatorIDList, memo.CreatorID)
+				creatorIDMap[memo.CreatorID] = true
+			}
 		}
 
-		// Batch load all users with a single query by getting all users and filtering
-		// Note: This is more efficient than N separate queries
-		for creatorID := range creatorIDs {
-			creator, err := s.Store.GetUser(ctx, &store.FindUser{ID: &creatorID})
-			if err == nil && creator != nil {
-				creatorMap[creatorID] = creator
-			}
+		// Batch load all users with a single query
+		users, err := s.Store.ListUsers(ctx, &store.FindUser{
+			IDList: creatorIDList,
+		})
+		if err != nil {
+			return "", lastModified, err
+		}
+		for _, creator := range users {
+			creatorMap[creator.ID] = creator
 		}
 	}
 

store/db/mysql/user.go

@@ -91,6 +91,19 @@ func (d *DB) ListUsers(ctx context.Context, find *store.FindUser) ([]*store.User
 	if v := find.ID; v != nil {
 		where, args = append(where, "`id` = ?"), append(args, *v)
 	}
+	if len(find.IDList) > 0 {
+		placeholders := make([]string, 0, len(find.IDList))
+		for range find.IDList {
+			placeholders = append(placeholders, "?")
+		}
+		where, args = append(where, fmt.Sprintf("`id` IN (%s)", strings.Join(placeholders, ", "))), append(args, func() []any {
+			list := make([]any, 0, len(find.IDList))
+			for _, id := range find.IDList {
+				list = append(list, id)
+			}
+			return list
+		}()...)
+	}
 	if v := find.Username; v != nil {
 		where, args = append(where, "`username` = ?"), append(args, *v)
 	}

store/db/postgres/user.go

@@ -94,6 +94,14 @@ func (d *DB) ListUsers(ctx context.Context, find *store.FindUser) ([]*store.User
 	if v := find.ID; v != nil {
 		where, args = append(where, "id = "+placeholder(len(args)+1)), append(args, *v)
 	}
+	if len(find.IDList) > 0 {
+		holders := make([]string, 0, len(find.IDList))
+		for range find.IDList {
+			holders = append(holders, placeholder(len(args)+1))
+			args = append(args, find.IDList[len(holders)-1])
+		}
+		where = append(where, fmt.Sprintf("id IN (%s)", strings.Join(holders, ", ")))
+	}
 	if v := find.Username; v != nil {
 		where, args = append(where, "username = "+placeholder(len(args)+1)), append(args, *v)
 	}

store/db/sqlite/user.go

@@ -95,6 +95,19 @@ func (d *DB) ListUsers(ctx context.Context, find *store.FindUser) ([]*store.User
 	if v := find.ID; v != nil {
 		where, args = append(where, "id = ?"), append(args, *v)
 	}
+	if len(find.IDList) > 0 {
+		placeholders := make([]string, 0, len(find.IDList))
+		for range find.IDList {
+			placeholders = append(placeholders, "?")
+		}
+		where, args = append(where, fmt.Sprintf("id IN (%s)", strings.Join(placeholders, ", "))), append(args, func() []any {
+			list := make([]any, 0, len(find.IDList))
+			for _, id := range find.IDList {
+				list = append(list, id)
+			}
+			return list
+		}()...)
+	}
 	if v := find.Username; v != nil {
 		where, args = append(where, "username = ?"), append(args, *v)
 	}

store/test/user_test.go

@@ -40,6 +40,36 @@ func TestUserStore(t *testing.T) {
 	ts.Close()
 }
 
+func TestUserListByIDList(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	ts := NewTestingStore(ctx, t)
+
+	// Create 5 users
+	var userIDs []int32
+	for i := 0; i < 5; i++ {
+		user, err := createTestingUserWithRole(ctx, ts, fmt.Sprintf("user_list_%d", i), store.RoleUser)
+		require.NoError(t, err)
+		userIDs = append(userIDs, user.ID)
+	}
+
+	// List users by IDList (3 out of 5)
+	targetIDs := userIDs[1:4]
+	users, err := ts.ListUsers(ctx, &store.FindUser{IDList: targetIDs})
+	require.NoError(t, err)
+	require.Equal(t, 3, len(users))
+
+	foundIDs := make(map[int32]bool)
+	for _, u := range users {
+		foundIDs[u.ID] = true
+	}
+	for _, id := range targetIDs {
+		require.True(t, foundIDs[id])
+	}
+
+	ts.Close()
+}
+
 func TestUserGetByID(t *testing.T) {
 	t.Parallel()
 	ctx := context.Background()

store/user.go

@@ -71,7 +71,9 @@ type UpdateUser struct {
 }
 
 type FindUser struct {
-	ID        *int32
+	ID     *int32
+	IDList []int32
+
 	RowStatus *RowStatus
 	Username  *string
 	Role      *Role