any one can help me here.how to improve query performance - mysql

Question

my query is running longer than 30 minutes .it is a simple query even it contains indexes also.we are unable to find why it was taking too much execution time and it effects on our entire db performance.
yesterday it ran around: 122.6mins
any one can help me here.how to improve query performance
This is my query:
SELECT tab1.customer_id,tab1.row_mod,tab1.row_create,tab1.event_id,tab1.event_type,
tab1.new_value,tab1.old_value FROM tab1 force index (tab1_n2)where customer_id >= 1 and customer_id
< 5000000 and (tab1.row_mod >= '2012-10-01') or (tab1.row_create >= '2012-10-01' and tab1.row_create < '2012-10-13');
Explain plan
+----+-------------+------------------+------+---------------------+------+---------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------------+------+---------------------+------+---------+------+----------+-------------+
| 1 | SIMPLE | tab1 | ALL | tab1_n2 | NULL | NULL | NULL | 18490530 | Using where |
+----+-------------+------------------+------+---------------------+------+---------+------+----------+-------------+
1 row in set (0.00 sec)
Table structure:
mysql> show create table tab1\G
*************************** 1. row ***************************
Table: tab1
Create Table: CREATE TABLE `tab1` (
`customer_id` int(11) NOT NULL,
`row_mod` datetime DEFAULT NULL,
`row_create` datetime DEFAULT NULL,
`event_id` int(11) DEFAULT NULL,
`event_type` varchar(45) DEFAULT NULL,
`new_value` varchar(255) DEFAULT NULL,
`old_value` varchar(255) DEFAULT NULL,
KEY `customer_id1` (`customer_id`),
KEY `new_value_n1` (`new_value`),
KEY `tab1_n1` (`row_create`),
KEY `tab1_n2` (`row_mod`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
Please help me how to tune it .even it having indexes also

Answer 1

Probably because you are using an index that does not make sense.
The row_mod condition is only one branch of the OR condition, so that index is not much help here. If you are forcing every lookup through the index without eliminating any rows, that could be a lot slower than a full table scan. Good rule of thumb is that an index should eliminate more than 90% of rows.
Try to do without the "force index" part.

Answer 2

Try using a UNION of the two conditions. That way each condition can use an index.
ALTER TABLE tab1 ADD INDEX idx_row_mod_customer_id (row_mod, customer_id);
ALTER TABLE tab1 ADD INDEX idx_row_create (row_create);
SELECT tab1.customer_id, tab1.row_mod, tab1.row_create, tab1.event_id, tab1.event_type,
tab1.new_value, tab1.old_value
FROM tab1
WHERE customer_id >= 1 and customer_id
< 5000000 AND tab1.row_mod >= '2012-10-01'
UNION
SELECT tab1.customer_id, tab1.row_mod, tab1.row_create, tab1.event_id, tab1.event_type,
tab1.new_value, tab1.old_value
FROM tab1
WHERE tab1.row_create >= '2012-10-01' AND tab1.row_create < '2012-10-13';
To optimise further, you could add all selected columns to both indices, saving MySQL from having to load the rows into memory. This will greatly increase the size of the indices, and therefore their memory requirement.