mysql> EXPLAIN SELECT sid FROM rf_fo.scald_atoms WHERE base_id ='493457' AND provider =  'rf_ressource_son' AND TYPE =  'audio';
+----+-------------+-------------+-------+------------------+---------+---------+-------+------+-------+
| id | select_type | table       | type  | possible_keys    | key     | key_len | ref   | rows | Extra |
+----+-------------+-------------+-------+------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | scald_atoms | const | base_id,provider | base_id | 767     | const |    1 |       |
+----+-------------+-------------+-------+------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT sid FROM rf_fo.scald_atoms WHERE base_id =493457 AND provider =  'rf_ressource_son' AND TYPE =  'audio';
+----+-------------+-------------+------+------------------+----------+---------+-------------+-------+-------------+
| id | select_type | table       | type | possible_keys    | key      | key_len | ref         | rows  | Extra       |
+----+-------------+-------------+------+------------------+----------+---------+-------------+-------+-------------+
|  1 | SIMPLE      | scald_atoms | ref  | base_id,provider | provider | 388     | const,const | 42202 | Using where |
+----+-------------+-------------+------+------------------+----------+---------+-------------+-------+-------------+
1 row in set (0.00 sec)