Optimization: add info about adding track_e_default.default_user_id index

documentation/optimization.html

@@ -258,6 +258,8 @@ ALTER TABLE c_lp_item_view ADD INDEX idx_clpiv_c_i_v (c_id, id, view_count);
         ALTER TABLE c_quiz_question ADD INDEX idx_cqq_cidid (c_id, id);
         -- If you have many messages with attachments
         ALTER TABLE message_attachment ADD index idx_msgat_msgid (message_id);
+        -- If you query audit logs from track_e_default on huge tables
+        ALTER TABLE track_e_default ADD INDEX idx_ted_uid (default_user_id);
     </pre>
 <hr />
 <h2><a name="3.Indexes-caching"></a>3. Indexes caching</h2>