Se eu fizer o que você fez acima, eu recebo
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 |
+----+-------------+-------+------+---------------+--------+---------+--------------------+------+-------------+
No entanto, se eu truncar, faça o seguinte carregamento de muitos dados (terminando com mais de 100 mil linhas):
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
Isso resulta em NÃO uma varredura de tabela:
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 |
+----+-------------+-------+--------+---------------+---------+---------+----------------------------+------+-------------+
O motivo está listado na página do manual Como o MySQL usa índices :
O motivo listado acima. No exemplo da sua pergunta, você tinha poucas linhas para fazer o uso do índice valer a pena. Portanto, o mecanismo db escolheu sua maneira supostamente (e provavelmente) mais rápida de não usar o índice em sua pequena tabela.
