以下查询使用 80% 或更多 CPU，可能需要 1 分钟以上才能完成。我的问题：我的查询有什么问题会导致这样的 CPU 使用吗？我可以通过优化 MySQL 服务器配置来减少 CPU 使用率和查询时间吗？
查询 1（loan_history 包含 260 万条记录）
SELECT officer, SUM(balance) as balance
FROM loan_history
WHERE bank_id = '1'
AND date ='2013-07-04'
AND officer IS NOT NULL
AND officer <> ''
GROUP BY officer
ORDER BY officer;
查询 2（loan_history 包含 260 万条记录）
SELECT SUM(weighted_interest_rate) as total
FROM (SELECT balance, tmp1.balance_sum,
(balance / tmp1.balance_sum * interest_rate) as weighted_interest_rate
FROM loan_history,
(SELECT SUM(balance) balance_sum FROM loan_history
WHERE date = '2013-07-04'
AND bank_id = '1') as tmp1
WHERE date = '2013-07-04'
AND bank_id = '1') tmp2
表信息：
CREATE TABLE `loan_history` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`bank_id` int(11) DEFAULT NULL,
`loan_purpose_id` int(11) DEFAULT NULL,
`date` date NOT NULL,
`credit_grade` varchar(5) COLLATE utf8_unicode_ci DEFAULT NULL,
`interest_rate` decimal(5,2) NOT NULL,
`officer` varchar(5) COLLATE utf8_unicode_ci DEFAULT NULL,
`balance` decimal(10,2) NOT NULL,
`start_date` date DEFAULT NULL,
`days_delinquent` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `IDX_9F5FE3F11C8FB41` (`bank_id`),
KEY `IDX_9F5FE3F6F593857` (`loan_purpose_id`),
KEY `date` (`date`),
KEY `credit_grade` (`credit_grade`),
KEY `officer` (`officer`),
KEY `start_date` (`start_date`),
KEY `days_delinquent` (`days_delinquent`),
KEY `interest_rate` (`interest_rate`),
KEY `balance` (`balance`),
CONSTRAINT `FK_9F5FE3F11C8FB41` FOREIGN KEY (`bank_id`) REFERENCES `bank` (`id`),
CONSTRAINT `FK_9F5FE3F6F593857` FOREIGN KEY (`loan_purpose_id`) REFERENCES `loan_purpose` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2630634 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
查询 1 解释：
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
| 1 | SIMPLE | loan_history | index_merge | IDX_9F5FE3F11C8FB41,date,officer | date,IDX_9F5FE3F11C8FB41 | 3,5 | NULL | 4829 | Using intersect(date,IDX_9F5FE3F11C8FB41); Using where; Using temporary; Using filesort |
查询 2 解释：
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 8236 |
| 2 | DERIVED | <derived3> | system | NULL | NULL | NULL | NULL | 1 |
| 2 | DERIVED | loan_history | index_merge | IDX_9F5FE3F11C8FB41,date | date,IDX_9F5FE3F11C8FB41 | 3,5 | NULL | 4829 | Using intersect(date,IDX_9F5FE3F11C8FB41); Using where; Using index |
| 3 | DERIVED | loan_history | index_merge | IDX_9F5FE3F11C8FB41,date | date,IDX_9F5FE3F11C8FB41 | 3,5 | NULL | 4829 | Using intersect(date,IDX_9F5FE3F11C8FB41); Using where; Using index |
我的.cnf 文件：
default-storage-engine=MyISAM
interactive_timeout=300
key_buffer_size=256M
key_cache_block_size=4096
max_heap_table_size=128M
max_join_size=1000000000
max_allowed_packet=32M
open_files_limit=4096
query_cache_size=256M
query_cache_limit=10240M
query_cache_type=1
table_cache=256
thread_cache_size=100
tmp_table_size=128M
wait_timeout=7800
max_user_connections=50
join_buffer_size=256K
sort_buffer_size=4M
read_rnd_buffer_size=1M
innodb_open_files=300
innodb_log_file_size=256M
innodb_log_buffer_size=8M
innodb_file_per_table=1
innodb_additional_mem_pool_size=20M
innodb_flush_log_at_trx_commit=0
innodb_flush_method=O_DIRECT
innodb_support_xa=0
innodb_thread_concurrency=0
innodb_buffer_pool_size=3000M