Why Index could refuse to work ?

Have you ever seen index which refused to be used even if there is every reason for it to work (from the glance view):

Shell

mysql> explain select * from article where article_id=10;
+—-+————-+———+——+—————+——+———+——+——-+————-+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | […]

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