Documentation: Add recommended index for c_item_property

documentation/optimization.html

@@ -247,6 +247,7 @@ ALTER TABLE c_lp_item_view ADD INDEX idx_clpiv_c_i_v (c_id, id, view_count);
         # Using the following queries might help:
         ALTER TABLE c_item_property ADD INDEX idx_cip_lasteditu (lastedit_user_id);
         ALTER TABLE c_item_property ADD INDEX idx_item_property_visibility (visibility);
+        ALTER TABLE c_item_property ADD INDEX idx_cip_ctv (c_id, tool(25), visibility);
         ALTER TABLE c_dropbox_person ADD INDEX idx_file(file_id);
 
         ALTER TABLE c_chat_connected ADD INDEX idx_user_course_session(user_id, c_id, session_id);