Acabei de me deparar com algo estranho. O base_id é um varchar(255). Quando faço um SELECT sem aspas, ele varre toda a tabela:
mysql> EXPLAIN SELECT sid FROM rf_fo.scald_atoms WHERE base_id = 493457;
+----+-------------+-------------+-------+---------------+---------+---------+------+-------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+------+-------+--------------------------+
| 1 | SIMPLE | scald_atoms | index | base_id | base_id | 767 | NULL | 84404 | Using where; Using index |
+----+-------------+-------------+-------+---------------+---------+---------+------+-------+--------------------------+
1 row in set (0.00 sec)
Com uma citação, está ok.
mysql> EXPLAIN SELECT sid FROM rf_fo.scald_atoms WHERE base_id = '493457';
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | SIMPLE | scald_atoms | const | base_id | base_id | 767 | const | 1 | Using index |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------------+
1 row in set (0.00 sec)