1 files changed, 40 insertions, 10 deletions
diff --git a/app/finders/autocomplete/users_finder.rb b/app/finders/autocomplete/users_finder.rb
index e7a24cde2bd..ddd68524a04 100644
--- a/app/finders/autocomplete/users_finder.rb
+++ b/app/finders/autocomplete/users_finder.rb
@@ -9,6 +9,7 @@ module Autocomplete
     # consistent and removes the need for implementing keyset pagination to
     # ensure good performance.
     LIMIT = 20
+    BATCH_SIZE = 1000
 
     attr_reader :current_user, :project, :group, :search,
       :author_id, :todo_filter, :todo_state_filter,
@@ -62,7 +63,19 @@ module Autocomplete
       # When changing the order of these method calls, make sure that
       # reorder_by_name() is called _before_ optionally_search(), otherwise
       # reorder_by_name will break the ORDER BY applied in optionally_search().
-      find_users
+      if project
+        project.authorized_users.union_with_user(author_id).merge(users_relation)
+      elsif group
+        find_group_users(group)
+      elsif current_user
+        users_relation
+      else
+        User.none
+      end.to_a
+    end
+
+    def users_relation
+      User
         .where(state: states)
         .non_internal
         .reorder_by_name
@@ -73,7 +86,6 @@ module Autocomplete
           todo_state: todo_state_filter
         )
         .limit(LIMIT)
-        .to_a
     end
     # rubocop: enable CodeReuse/ActiveRecord
 
@@ -85,15 +97,33 @@ module Autocomplete
       author_id.present? && current_user
     end
 
-    def find_users
-      if project
-        project.authorized_users.union_with_user(author_id)
-      elsif group
-        ::Autocomplete::GroupUsersFinder.new(group: group).execute # rubocop: disable CodeReuse/Finder
-      elsif current_user
-        User.all
+    def find_group_users(group)
+      if Feature.enabled?(:group_users_autocomplete_using_batch_reduction, current_user)
+        members_relation = ::Autocomplete::GroupUsersFinder.new(group: group, members_relation: true).execute # rubocop: disable CodeReuse/Finder
+
+        user_ids = Set.new
+        members_relation.each_batch(of: BATCH_SIZE) do |relation|
+          user_ids += relation.pluck_user_ids
+        end
+
+        user_relations = []
+        user_ids.to_a.in_groups_of(BATCH_SIZE, false) do |batch_user_ids|
+          user_relations << users_relation.id_in(batch_user_ids)
+        end
+
+        # When there is more than 1 batch, we need to apply users_relation again on
+        # the top results per-batch so that we return results in the correct order.
+        if user_relations.empty?
+          User.none
+        elsif user_relations.one?
+          user_relations.first
+        else
+          # We pluck the ids per batch so that we don't send an unbounded number of ids in one query
+          user_ids = user_relations.flat_map(&:pluck_primary_key)
+          users_relation.id_in(user_ids)
+        end
       else
-        User.none
+        ::Autocomplete::GroupUsersFinder.new(group: group).execute.merge(users_relation) # rubocop: disable CodeReuse/Finder
       end
     end