Improve pagination of users in admin panel

When there are a large number of users the pagination query in the admin
panel users list times out.

This commit changes the query to an approximate and paginates without
counts if it is over the limit.

Changelog: performance

app/controllers/admin/users_controller.rb

@@ -10,12 +10,15 @@ class Admin::UsersController < Admin::ApplicationController
 
   feature_category :users
 
+  PAGINATION_WITH_COUNT_LIMIT = 1000
+
   def index
     @users = User.filter_items(params[:filter]).order_name_asc
     @users = @users.search_with_secondary_emails(params[:search_query]) if params[:search_query].present?
     @users = users_with_included_associations(@users)
     @users = @users.sort_by_attribute(@sort = params[:sort])
     @users = @users.page(params[:page])
+    @users = @users.without_count if paginate_without_count?
 
     @cohorts = load_cohorts
 
@@ -228,6 +231,12 @@ class Admin::UsersController < Admin::ApplicationController
 
   protected
 
+  def paginate_without_count?
+    counts = Gitlab::Database::Count.approximate_counts([User])
+
+    counts[User] > PAGINATION_WITH_COUNT_LIMIT
+  end
+
   def users_with_included_associations(users)
     users.includes(:authorized_projects) # rubocop: disable CodeReuse/ActiveRecord
   end

app/views/admin/users/_users.html.haml

@@ -83,6 +83,6 @@
 
     = render partial: 'admin/users/user', collection: @users
 
-= paginate @users, theme: "gitlab"
+= paginate_collection @users
 
 = render partial: 'admin/users/modals'

changelogs/unreleased/328519-fix-timeouts-when-paginating-issues-in-admin-panel.yml

+---
+title: Improve pagination of users in the admin panel
+merge_request: 59884
+author:
+type: performance

spec/controllers/admin/users_controller_spec.rb

@@ -34,6 +34,29 @@ RSpec.describe Admin::UsersController do
       let(:target_id) { 'i_analytics_cohorts' }
       let(:request_params) { { tab: 'cohorts' } }
     end
+
+    context 'pagination' do
+      context 'when number of users is over the pagination limit' do
+        before do
+          stub_const('Admin::UsersController::PAGINATION_WITH_COUNT_LIMIT', 5)
+          allow(Gitlab::Database::Count).to receive(:approximate_counts).with([User]).and_return({ User => 6 })
+        end
+
+        it 'marks the relation for pagination without counts' do
+          get :index
+
+          expect(assigns(:users)).to be_a(Kaminari::PaginatableWithoutCount)
+        end
+      end
+
+      context 'when number of users is below the pagination limit' do
+        it 'marks the relation for pagination with counts' do
+          get :index
+
+          expect(assigns(:users)).not_to be_a(Kaminari::PaginatableWithoutCount)
+        end
+      end
+    end
   end
 
   describe 'GET :id' do