Documentation: Add optimization note on message_attachment index addition - refs BT#19984

documentation/optimization.html

@@ -256,6 +256,8 @@ ALTER TABLE c_lp_item_view ADD INDEX idx_clpiv_c_i_v (c_id, id, view_count);
         ALTER TABLE c_chat_connected ADD INDEX idx_course_group(c_id, to_group_id);
         -- For big online exams, this index reduces the load considerably
         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);
     </pre>
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 <h2><a name="3.Indexes-caching"></a>3. Indexes caching</h2>