Question

Here are the sql commands I’ve done, and I don’t understand why there’s so few difference between without the key (~1.45 s), and with the key (~1.15 s):

mysql> show create table devis;
| devis | CREATE TABLE `devis` (
  `id` bigint(20) unsigned NOT NULL auto_increment,
  ...blabla...
  `date_v_creation` datetime default NULL,
  `date_v_fin` datetime default NULL,
  ...blabla...
  `etat` tinyint(4) default NULL,
  `etat_date` datetime default NULL,
  PRIMARY KEY  (`id`),
  KEY `date_v_creation` (`date_v_creation`),
  KEY `date_v_fin` (`date_v_fin`),
  ...blabla...
) ENGINE=InnoDB AUTO_INCREMENT=3714317 DEFAULT CHARSET=utf8 |

mysql> select date_v_creation from devis
where etat=3
and date_v_fin is null
and TO_DAYS(NOW()) - TO_DAYS(date_v_creation) > 54;

+---------------------+
| date_v_creation     |
+---------------------+
| 2011-10-30 21:44:54 | 
| ...blabla...        | 
| 2011-10-30 21:48:05 | 
+---------------------+
216 rows in set (1.45 sec)

mysql> alter table devis add key( date_v_fin, etat);

mysql> select date_v_creation from devis
where etat=3
and date_v_fin is null
and TO_DAYS(NOW()) - TO_DAYS(date_v_creation) > 54;

+---------------------+
| date_v_creation     |
+---------------------+
| 2011-10-30 21:44:54 | 
| ...blabla...        | 
| 2011-10-30 21:48:05 | 
+---------------------+
216 rows in set (1.13 sec)

If I do it a lot of times, it’s always between (1.05 sec) and (1.15 sec).

There have been a lot of times where I’ve added keys to have a shorter response time, and when it didn’t work, the time didn’t change, and when it worked, the time change to a factor of 10 to the least.

Am I doing something wrong, missing something or is this normal t gain so few?

Thanks a lot!

Update

Here’s the query that solved my problem but I still don’t get why it solved it.

select date_v_creation
from devis
where etat=3
  and date_v_creation < NOW() - INTERVAL 54 DAY
  and date_v_fin is null;   

Here’s the explain of my “bad” query:

mysql> EXPLAIN select date_v_creation from devis where etat=3 and date_v_fin is null and TO_DAYS(NOW()) - TO_DAYS(date_v_creation) > 54;
+----+-------------+-------+------+-------------------------+------------+---------+-------+-------+-------------+
| id | select_type | table | type | possible_keys           | key        | key_len | ref   | rows  | Extra       |
+----+-------------+-------+------+-------------------------+------------+---------+-------+-------+-------------+
|  1 | SIMPLE      | devis | ref  | date_v_fin,date_v_fin_2 | date_v_fin | 9       | const | 15913 | Using where |
+----+-------------+-------+------+-------------------------+------------+---------+-------+-------+-------------+
1 row in set (0.34 sec)

Here’s the explain of the “good”, optimized query:

mysql> explain select date_v_creation from devis where etat=3   and date_v_creation < NOW() - INTERVAL 54 DAY   and date_v_fin is null;
+----+-------------+-------+-------+-----------------------------------------+-----------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys                           | key             | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+-----------------------------------------+-----------------+---------+------+------+-------------+
|  1 | SIMPLE      | devis | range | date_v_fin,date_v_creation,date_v_fin_2 | date_v_creation | 9       | NULL | 1458 | Using where |
+----+-------------+-------+-------+-----------------------------------------+-----------------+---------+------+------+-------------+
1 row in set (0.00 sec)

Answer 1

You have basically two problems:

1. There is no useful key for the query: useful here is the key that includes index on the field with equation, and then with the comparison. That is you need a key on (etat, date_v_creation)

2. the index on date_v_creation currently can not be used, because the comparison is on a function of the field value instead of the value itself. That’s why the key on date_v_creation did not help. SO you should rewrite the query as follows:

   select date_v_creation
   from devis
   where etat=3
     and date_v_creation < NOW() - INTERVAL 54 DAY
     and date_v_fin is null;