EXPLAIN SELECT messageblk, is_header FROM dbmail_messageblks WHERE physmessage_id = 1314716 ORDER BY messageblk_idnr:

*** row 1 ***
          table:  dbmail_messageblks
           type:  index
  possible_keys:  physmessage_id_is_header_index
            key:  PRIMARY
        key_len:  8
            ref:  NULL
           rows:  2163086
          Extra:  Using where


show create table dbmail_messageblks;
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table              | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| dbmail_messageblks | CREATE TABLE `dbmail_messageblks` (
  `messageblk_idnr` bigint(21) NOT NULL AUTO_INCREMENT,
  `physmessage_id` bigint(21) NOT NULL DEFAULT '0',
  `messageblk` longblob NOT NULL,
  `blocksize` bigint(21) NOT NULL DEFAULT '0',
  `is_header` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`messageblk_idnr`),
  KEY `physmessage_id_is_header_index` (`physmessage_id`,`is_header`)
) ENGINE=InnoDB AUTO_INCREMENT=2466226 DEFAULT CHARSET=latin1 /*!50100 PARTITION BY HASH (messageblk_idnr) PARTITIONS 16  */ |
+--------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+


show index from dbmail_messageblks;
+--------------------+------------+--------------------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+
| Table              | Non_unique | Key_name                       | Seq_in_index | Column_name     | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+--------------------+------------+--------------------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+
| dbmail_messageblks |          0 | PRIMARY                        |            1 | messageblk_idnr | A         |     2098848 |     NULL | NULL   |      | BTREE      |         |
| dbmail_messageblks |          1 | physmessage_id_is_header_index |            1 | physmessage_id  | A         |     2098848 |     NULL | NULL   |      | BTREE      |         |
| dbmail_messageblks |          1 | physmessage_id_is_header_index |            2 | is_header       | A         |     2098848 |     NULL | NULL   |      | BTREE      |         |
+--------------------+------------+--------------------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+
3 rows in set (54.51 sec)

02:25:23> explain partitions SELECT messageblk, is_header FROM dbmail_messageblks WHERE physmessage_id = 1314716 ORDER BY messageblk_idnr;
+----+-------------+--------------------+-------------------------------------------------------+-------+--------------------------------+---------+---------+------+---------+-------------+
| id | select_type | table              | partitions                                            | type  | possible_keys                  | key     | key_len | ref  | rows    | Extra       |
+----+-------------+--------------------+-------------------------------------------------------+-------+--------------------------------+---------+---------+------+---------+-------------+
|  1 | SIMPLE      | dbmail_messageblks | p0,p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p11,p12,p13,p14,p15 | index | physmessage_id_is_header_index | PRIMARY | 8       | NULL | 2150055 | Using where |
+----+-------------+--------------------+-------------------------------------------------------+-------+--------------------------------+---------+---------+------+---------+-------------+
1 row in set (0.00 sec)

02:25:33> select MOD(1314716,16);
+-----------------+
| MOD(1314716,16) |
+-----------------+
|              12 |
+-----------------+
1 row in set (0.00 sec)