Si j'exécute ce que vous avez fait ci-dessus, j'obtiens
EXPLAIN select * from stats_clicked s
join visitor v on s.visitor_id=v.id
+----+-------------+-------+------+---------------+--------+---------+--------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+--------+---------+--------------------+------+-------------+
| 1 | SIMPLE | v | ALL | PRIMARY | NULL | NULL | NULL | 15 | NULL |
| 1 | SIMPLE | s | ref | index3,index1 | index3 | 4 | so_gibberish2.v.id | 1 | Using index |
+----+-------------+-------+------+---------------+--------+---------+--------------------+------+-------------+
Cependant, si je tronque, je fais le chargement suivant d'un grand nombre de données (se terminant avec plus de 100 000 lignes) :
truncate table visitor;
insert into visitor (`visited_time`) values
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944),
(1467122944),(1467122944),(1467122944);
insert into visitor (`visited_time`) values
(1467122945),(1467122945),(1467122945),
(1467122945),(1467122945),(1467122945),
(1467122945),(1467122945),(1467122945),
(1467122945),(1467122945),(1467122945),
(1467122945),(1467122945),(1467122945),
insert into visitor (`visited_time`) values
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946),
(1467122946),(1467122946),(1467122946);
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
insert visitor(`visited_time`) select `visited_time` from visitor;
select count(*) from visitor;
-- 104448 rows
Il en résulte PAS un parcours de table :
EXPLAIN select * from stats_clicked s
join visitor v on s.visitor_id=v.id;
+----+-------------+-------+--------+---------------+---------+---------+----------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+----------------------------+------+-------------+
| 1 | SIMPLE | s | index | index3,index1 | index3 | 9 | NULL | 22 | Using index |
| 1 | SIMPLE | v | eq_ref | PRIMARY | PRIMARY | 4 | so_gibberish2.s.visitor_id | 1 | NULL |
+----+-------------+-------+--------+---------------+---------+---------+----------------------------+------+-------------+
La raison est répertoriée dans la page de manuel Comment MySQL utilise les index :
La raison étant énumérée ci-dessus. Dans l'exemple de votre question, vous aviez trop peu de lignes pour que l'utilisation de l'index en vaille la peine. Le moteur de base de données a donc choisi son moyen prétendument (et probablement) plus rapide de ne pas utiliser l'index sur votre petite table.
