Fix slow events query for followed users

Changelog: performance

app/finders/user_recent_events_finder.rb

@@ -46,6 +46,20 @@ class UserRecentEventsFinder
       .order_created_desc)
   end
 
+  # rubocop: disable CodeReuse/ActiveRecord
+  def execute_optimized_multi(users)
+    Gitlab::Pagination::Keyset::InOperatorOptimization::QueryBuilder.new(
+      scope: Event.reorder(:id),
+      array_scope: User.select(:id).where(id: users),
+      array_mapping_scope: -> (author_id_expression) { Event.where(Event.arel_table[:author_id].eq(author_id_expression)) },
+      finder_query: -> (id_expression) { Event.where(Event.arel_table[:id].eq(id_expression)) }
+    )
+    .execute
+    .limit(limit)
+    .offset(params[:offset] || 0)
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
+
   # rubocop: disable CodeReuse/ActiveRecord
   def execute_multi
     users = []
@@ -55,8 +69,12 @@ class UserRecentEventsFinder
 
     return Event.none if users.empty?
 
+    if event_filter.filter == EventFilter::ALL
+      execute_optimized_multi(users)
+    else
       event_filter.apply_filter(Event.where(author: users).limit_recent(limit, params[:offset] || 0))
     end
+  end
   # rubocop: enable CodeReuse/ActiveRecord
 
   # rubocop: disable CodeReuse/ActiveRecord

db/post_migrate/20211217145923_add_index_to_events_on_author_id_and_action_and_id.rb

+# frozen_string_literal: true
+
+class AddIndexToEventsOnAuthorIdAndActionAndId < Gitlab::Database::Migration[1.0]
+  INDEX_NAME = 'index_events_on_author_id_and_action_and_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :events, [:author_id, :action, :id], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :events, INDEX_NAME
+  end
+end

db/schema_migrations/20211217145923

+2e35347592530f2a73e1cd75871438e29d277a206f621989e2c897fc587b6f5d
\ No newline at end of file

db/structure.sql

@@ -26024,6 +26024,8 @@ CREATE INDEX index_events_author_id_project_id_action_target_type_created_at ON
 
 CREATE INDEX index_events_on_action ON events USING btree (action);
 
+CREATE INDEX index_events_on_author_id_and_action_and_id ON events USING btree (author_id, action, id);
+
 CREATE INDEX index_events_on_author_id_and_created_at ON events USING btree (author_id, created_at);
 
 CREATE INDEX index_events_on_author_id_and_created_at_merge_requests ON events USING btree (author_id, created_at) WHERE ((target_type)::text = 'MergeRequest'::text);

spec/finders/user_recent_events_finder_spec.rb

@@ -59,14 +59,46 @@ RSpec.describe UserRecentEventsFinder do
         expect(events.size).to eq(6)
       end
 
+      context 'selected events' do
+        let!(:push_event)                  { create(:push_event, project: public_project, author: project_owner) }
+        let!(:push_event_second_user)      { create(:push_event, project: public_project_second_user, author: second_user) }
+
+        it 'only includes selected events (PUSH) from all users', :aggregate_failures do
+          event_filter = EventFilter.new(EventFilter::PUSH)
+          events = described_class.new(current_user, [project_owner, second_user], event_filter, params).execute
+
+          expect(events).to contain_exactly(push_event, push_event_second_user)
+        end
+      end
+
       it 'does not include events from users with private profile', :aggregate_failures do
         allow(Ability).to receive(:allowed?).and_call_original
         allow(Ability).to receive(:allowed?).with(current_user, :read_user_profile, second_user).and_return(false)
 
         events = described_class.new(current_user, [project_owner, second_user], nil, params).execute
 
-        expect(events).to include(private_event, internal_event, public_event)
-        expect(events.size).to eq(3)
+        expect(events).to eq([private_event, internal_event, public_event])
+      end
+
+      context 'with pagination params' do
+        using RSpec::Parameterized::TableSyntax
+
+        where(:limit, :offset, :ordered_expected_events) do
+          nil | nil   | lazy { [private_event, internal_event, public_event, private_event_second_user, internal_event_second_user, public_event_second_user] }
+          2   | nil   | lazy { [private_event, internal_event] }
+          nil | 4     | lazy { [internal_event_second_user, public_event_second_user] }
+          2   | 2     | lazy { [public_event, private_event_second_user] }
+        end
+
+        with_them do
+          let(:params) { { limit: limit, offset: offset }.compact }
+
+          it 'returns paginated events sorted by id' do
+            events = described_class.new(current_user, [project_owner, second_user], nil, params).execute
+
+            expect(events).to eq(ordered_expected_events)
+          end
+        end
       end
     end