ThreadStore: Remove quadratic complexity for participants population (#18755)

* ThreadStore: Remove quadratic complexity for participants population

While populating the participants, we would traverse the global list
twice to get the users for that thread. For threads with large number
of users, this can take considerable time. We replace that with a map
to change it to O(n).

While here, we improve memory usage of this code by pre-allocating
slices and maps. And also add some comments to help readers.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4?c=c9aa17f3-b918-4a78-86aa-01562e418634

```release-note
NONE
```

* Apply in post store as well

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/post_store.go

@@ -819,33 +819,31 @@ func (s *SqlPostStore) prepareThreadedResponse(posts []*postWithExtra, extended,
 			}
 		}
 	}
-	var users []*model.User
+	// usersMap is the global profile map of all participants from all threads.
+	usersMap := make(map[string]*model.User, len(userIds))
 	if extended {
-		var err error
-		users, err = s.User().GetProfileByIds(context.Background(), userIds, &store.UserGetByIdsOpts{}, true)
+		users, err := s.User().GetProfileByIds(context.Background(), userIds, &store.UserGetByIdsOpts{}, true)
 		if err != nil {
 			return nil, err
 		}
+		for _, user := range users {
+			usersMap[user.Id] = user
+		}
 	} else {
 		for _, userId := range userIds {
-			users = append(users, &model.User{Id: userId})
+			usersMap[userId] = &model.User{Id: userId}
 		}
 	}
+
 	processPost := func(p *postWithExtra) error {
 		p.Post.ReplyCount = p.ThreadReplyCount
 		if p.IsFollowing != nil {
 			p.Post.IsFollowing = model.NewBool(*p.IsFollowing)
 		}
-		for _, th := range p.ThreadParticipants {
-			var participant *model.User
-			for _, u := range users {
-				if u.Id == th {
-					participant = u
-					break
-				}
-			}
-			if participant == nil {
-				return errors.New("cannot find thread participant with id=" + th)
+		for _, userID := range p.ThreadParticipants {
+			participant, ok := usersMap[userID]
+			if !ok {
+				return errors.New("cannot find thread participant with id=" + userID)
 			}
 			p.Post.Participants = append(p.Post.Participants, participant)
 		}

store/sqlstore/thread_store.go

@@ -291,7 +291,10 @@ func (s *SqlThreadStore) GetThreadsForUser(userId, teamId string, opts model.Get
 	}
 	totalUnreadThreads := totalUnreadThreadsResult.Data.(int64)
 
+	// userIds is the de-duped list of participant ids from all threads.
 	var userIds []string
+	// userIdMap is the map of participant ids from all threads.
+	// Used to generate userIds
 	userIdMap := map[string]bool{}
 
 	result := &model.Threads{
@@ -315,30 +318,30 @@ func (s *SqlThreadStore) GetThreadsForUser(userId, teamId string, opts model.Get
 				}
 			}
 		}
-		var users []*model.User
+		// usersMap is the global profile map of all participants from all threads.
+		usersMap := make(map[string]*model.User, len(userIds))
 		if opts.Extended {
-			var err error
-			users, err = s.User().GetProfileByIds(context.Background(), userIds, &store.UserGetByIdsOpts{}, true)
+			users, err := s.User().GetProfileByIds(context.Background(), userIds, &store.UserGetByIdsOpts{}, true)
 			if err != nil {
 				return nil, errors.Wrapf(err, "failed to get threads for user id=%s", userId)
 			}
+			for _, user := range users {
+				usersMap[user.Id] = user
+			}
 		} else {
 			for _, userId := range userIds {
-				users = append(users, &model.User{Id: userId})
+				usersMap[userId] = &model.User{Id: userId}
 			}
 		}
 
+		result.Threads = make([]*model.ThreadResponse, 0, len(threads))
 		for _, thread := range threads {
-			var participants []*model.User
+			participants := make([]*model.User, 0, len(thread.Participants))
+			// We get the user profiles for only a single thread filtered from the
+			// global users map.
 			for _, participantId := range thread.Participants {
-				var participant *model.User
-				for _, u := range users {
-					if u.Id == participantId {
-						participant = u
-						break
-					}
-				}
-				if participant == nil {
+				participant, ok := usersMap[participantId]
+				if !ok {
 					return nil, errors.New("cannot find thread participant with id=" + participantId)
 				}
 				participants = append(participants, participant)