some benchmark

mysql> describe smalltable;
+------------------+--------------------------------------------+------+-----+---------------------+----------------+
| Field            | Type                                       | Null | Key | Default             | Extra          |
+------------------+--------------------------------------------+------+-----+---------------------+----------------+
| id               | int(11) unsigned                           | NO   | PRI | NULL                | auto_increment |
|                  | int(11) unsigned                           | NO   | MUL | 0                   |                |
|                  | int(11) unsigned                           | NO   | MUL | 0                   |                |
| varchar255       | varchar(255)                               | NO   | UNI |                     |                |
|                  | bigint(20) unsigned                        | NO   |     | NULL                |                |
|                  | datetime                                   | NO   | MUL | 0000-00-00 00:00:00 |                |
|                  | datetime                                   | NO   |     | 0000-00-00 00:00:00 |                |
|                  | enum('FREE','DONE','FAILED','IN_PROGRESS') | NO   | MUL | NULL                |                |
|                  | int(11)                                    | NO   |     | NULL                |                |
|                  | int(2)                                     | NO   | MUL | -1                  |                |
|                  | tinyint(3) unsigned                        | NO   |     | 0                   |                |
+------------------+--------------------------------------------+------+-----+---------------------+----------------+
11 rows in set (0.00 sec)

mysql> set profiling =1;
Query OK, 0 rows affected (0.00 sec)

mysql> select count(*) from smalltable;
+-----------+
| count(*)  |
+-----------+
| 249711281 |
+-----------+
1 row in set (2 min 30.98 sec)

mysql> select varchar255 from smalltable where id = 178245354;
+----------------------------------------------------------------------+
| varchar255                                                           |
+----------------------------------------------------------------------+
| xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx |
+----------------------------------------------------------------------+
1 row in set (0.01 sec)

mysql> select id from smalltable where varchar255='xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
+-----------+
| id        |
+-----------+
| 178245354 |
+-----------+
1 row in set (0.00 sec)

mysql> select varchar255 from smalltable where id = 178245354;
+----------------------------------------------------------------------+
| varchar255                                                           |
+----------------------------------------------------------------------+
| xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx |
+----------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select varchar255 from smalltable where id = 78245354;
+---------------------------------------------------------------------------------------------------------------+
| varchar255                                                                                                    |
+---------------------------------------------------------------------------------------------------------------+
| wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww |
+---------------------------------------------------------------------------------------------------------------+
1 row in set (0.12 sec)

mysql> select varchar255 from smalltable where id = 128245354;
+---------------------------------------------------------------------------+
| varchar255                                                                |
+---------------------------------------------------------------------------+
| qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq |
+---------------------------------------------------------------------------+
1 row in set (0.02 sec)

mysql> select id from smalltable where varchar255='xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
+-----------+
| id        |
+-----------+
| 178245354 |
+-----------+
1 row in set (0.00 sec)

mysql> select id from smalltable where varchar255='wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww';
+----------+
| id       |
+----------+
| 78245354 |
+----------+
1 row in set (0.02 sec)

mysql> select id from smalltable where varchar255='qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq';
+-----------+
| id        |
+-----------+
| 128245354 |
+-----------+
1 row in set (0.00 sec)

mysql> set profiling=0;
Query OK, 0 rows affected (0.00 sec)

mysql> show profiles;
+----------+--------------+------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query_ID | Duration     | Query                                                                                                                                                      |
+----------+--------------+------------------------------------------------------------------------------------------------------------------------------------------------------------+
|        1 | 150.97058275 | select count(*) from smalltable                                                                                                                            |
|        2 |   0.00673750 | select varchar255 from smalltable where id = 178245354                                                                                                     |
|        3 |   0.00032600 | select id from smalltable where varchar255='xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'                                          |
|        4 |   0.00024225 | select varchar255 from smalltable where id = 178245354                                                                                                     |
|        5 |   0.12738300 | select varchar255 from smalltable where id = 78245354                                                                                                      |
|        6 |   0.01478350 | select varchar255 from smalltable where id = 128245354                                                                                                     |
|        7 |   0.00030250 | select id from smalltable where varchar255='xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'                                          |
|        8 |   0.01827275 | select id from smalltable where varchar255='wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww' |
|        9 |   0.00021725 | select id from smalltable where varchar255='qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq'                                     |
+----------+--------------+------------------------------------------------------------------------------------------------------------------------------------------------------------+
9 rows in set (0.00 sec)