Partition behaiour seems abnormal in range partition

[Rahul K K]

I have created a table using mysql partition using range and have inserted millions of data.



CREATE TABLE `PART_SAMPLE ` (
`TRANSACTION_ID` bigint(25) NOT NULL AUTO_INCREMENT,
`TASK_ID` int(11) DEFAULT NULL,
`STATUS_CODE` int(10) DEFAULT NULL,
`FIELD10` int(5) DEFAULT NULL,
KEY `TXN_ID` (`TRANSACTION_ID`),
KEY `TASK_IDX` (`TASK_ID`),
KEY `id_idx_task_status` (`TASK_ID`,`STATUS_CODE`),
KEY `IDX_STATUS` (`STATUS_CODE`),
KEY `Fld_idx` (`FIELD10`)
) ENGINE=MyISAM AUTO_INCREMENT=12249932 DEFAULT CHARSET=latin1
/*!50100 PARTITION BY RANGE (FIELD10)
(PARTITION p0 VALUES LESS THAN (0) ENGINE = MyISAM,
PARTITION p1 VALUES LESS THAN (1) ENGINE = MyISAM,
PARTITION p2 VALUES LESS THAN (2) ENGINE = MyISAM,
PARTITION p3 VALUES LESS THAN (3) ENGINE = MyISAM,
PARTITION p4 VALUES LESS THAN (4) ENGINE = MyISAM,
PARTITION p5 VALUES LESS THAN (5) ENGINE = MyISAM,
PARTITION p6 VALUES LESS THAN (6) ENGINE = MyISAM,
PARTITION p7 VALUES LESS THAN (7) ENGINE = MyISAM,
PARTITION p8 VALUES LESS THAN (8) ENGINE = MyISAM,
PARTITION p9 VALUES LESS THAN (9) ENGINE = MyISAM,
PARTITION p10 VALUES LESS THAN MAXVALUE ENGINE = MyISAM) */

**Each Field10(0-10) value is having 3 million data each. But when am executing a select query as this**


select TASK_ID,STATUS_CODE,count(*) from PART_SAMPLE where FIELD10=X group by TASK_ID,STATUS_CODE;

x can be any value in the partition

for x value 0,2,5,8 it is taking only 10 seconds to retrive result but for rest it is taking abount 50s to rerive the result. As per my understating since data is same for all Fields almost same time has to be taken for any Field10 value. Why this time difference is coming. Is it because the way i have used partitioning is wrong or some thing else?