group member count optimization

db/migrate/20190722003541_add_member_count_to_groups.rb

@@ -0,0 +1,10 @@
+class AddMemberCountToGroups < ActiveRecord::Migration[5.2]
+  def up
+    add_column :groups, :member_count, :integer
+    change_column_default :groups, :member_count, 0
+  end
+
+  def down
+    remove_column :groups, :member_count
+  end
+end

db/migrate/20190722003649_backfill_add_member_count_to_groups.rb

@@ -0,0 +1,10 @@
+class BackfillAddMemberCountToGroups < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    Group.in_batches do |relation|
+      relation.update_all member_count: 0
+      sleep(0.1)
+    end
+  end
+end