我在MySQL中有一个使用多个连接的查询，此时它运行缓慢 - 平均需要大约35秒才能运行。
查询是：
SELECT t.id,
CASE t.emp_accepted
WHEN '1' THEN 'No'
WHEN '0' THEN 'Yes'
END AS accepted,
e.department,
e.works_id,
e.first_name,
e.sur_name,
e.job_title,
e.job_status,
e.site_id,
e.manager,
d1.department_name AS dept_name,
d2.department_name AS sub_dept_name,
temp_hours_worked.hours AS hours,
s.office_name AS site_name,
CONCAT(e2.first_name, ' ', e2.sur_name) AS manager_name,
CONCAT(e3.first_name, ' ', e3.sur_name) AS validated_by
FROM time t
LEFT JOIN employee e
ON t.employee_id = e.employee_id
LEFT JOIN departments d1
ON e.department = d1.id
LEFT JOIN departments d2
ON e.sub_department = d2.id
LEFT JOIN site s
ON e.site_id = s.id
LEFT JOIN employee e2
ON e.manager = e2.id
LEFT JOIN employee e3
ON t.manager_id = e3.id
LEFT JOIN temp_hours_worked
ON temp_hours_worked.week_beginning = t.week_beginning
AND temp_hours_worked.employee_id = t.employee_id
AND temp_hours_worked.company_id=?
WHERE t.company_id = ?;
说明：
+----+-------------+-------------------+--------+---------------+-------------+---------+-----------------------------------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------------+--------+---------------+-------------+---------+-----------------------------------------+------+-------+
| 1 | SIMPLE | t | ref | company_id | company_id | 4 | const | 5566 | |
| 1 | SIMPLE | e | ref | employee_id | employee_id | 4 | DBNAME.t.employee_id | 1 | |
| 1 | SIMPLE | d1 | eq_ref | PRIMARY | PRIMARY | 4 | DBNAME.e.department | 1 | |
| 1 | SIMPLE | d2 | eq_ref | PRIMARY | PRIMARY | 4 | DBNAME.e.sub_department | 1 | |
| 1 | SIMPLE | s | eq_ref | PRIMARY | PRIMARY | 4 | DBNAME.e.site_id | 1 | |
| 1 | SIMPLE | e2 | eq_ref | PRIMARY | PRIMARY | 4 | DBNAME.e.manager | 1 | |
| 1 | SIMPLE | e3 | eq_ref | PRIMARY | PRIMARY | 4 | DBNAME.t.manager_id | 1 | |
| 1 | SIMPLE | temp_hours_worked | ref | company_id | company_id | 4 | const | 5566 | |
+----+-------------+-------------------+--------+---------------+-------------+---------+-----------------------------------------+------+-------+
MySQL版本是在Centos 6.5 64位上运行的5.5.31，服务器是8核，4GB RAM和SSD磁盘。盒子上的平均负载是：
load average: 0.24, 0.29, 0.29
和空闲记忆显示为：
total used free shared buffers cached
Mem: 3880 3067 813 0 177 1065
-/+ buffers/cache: 1825 2055
Swap: 1023 0 1023
磁盘空间正常：
Filesystem Size Used Avail Use% Mounted on
/dev/xvda1 45G 11G 32G 26% /
tmpfs 1.9G 0 1.9G 0% /dev/shm
/usr/tmpDSK 1008M 51M 907M 6% /tmp
hdparm -Tt / dev / xvda1
的输出 Timing cached reads: 12538 MB in 1.99 seconds = 6297.76 MB/sec
Timing buffered disk reads: 826 MB in 3.00 seconds = 275.27 MB/sec
的my.cnf：
[mysql]
# CLIENT #
port = 3306
socket = /var/lib/mysql/mysql.sock
[mysqld]
local-infile=0
# GENERAL #
user = mysql
default_storage_engine = InnoDB
socket = /var/lib/mysql/mysql.sock
pid_file = /var/lib/mysql/mysql.pid
# MyISAM #
key_buffer_size = 32M
myisam_recover = FORCE,BACKUP
# SAFETY #
max_allowed_packet = 16M
max_connect_errors = 1000000
skip_name_resolve
#sql_mode = NO_AUTO_CREATE_USER,NO_AUTO_VALUE_ON_ZERO,NO_ENGINE_SUBSTITUTION,NO_ZERO_DATE,NO_ZERO_IN_DATE,ONLY_FULL_GROUP_BY
sysdate_is_now = 1
innodb = FORCE
#innodb_strict_mode = 1
# DATA STORAGE #
datadir = /var/lib/mysql/
# BINARY LOGGING #
log_bin = /var/lib/mysql/mysql-bin
expire_logs_days = 14
sync_binlog = 1
# CACHES AND LIMITS #
tmp_table_size = 32M
max_heap_table_size = 32M
query_cache_type = 0
query_cache_size = 0
max_connections = 500
thread_cache_size = 50
open_files_limit = 65535
table_definition_cache = 4096
table_open_cache = 4096
# INNODB #
innodb_flush_method = O_DIRECT
innodb_log_files_in_group = 2
innodb_log_file_size = 128M
innodb_flush_log_at_trx_commit = 1
innodb_file_per_table = 1
innodb_buffer_pool_size = 1456M
# LOGGING #
log_error = /var/lib/mysql/mysql-error.log
#log_queries_not_using_indexes = 1
slow_query_log = 1
slow_query_log_file = /var/lib/mysql/mysql-slow.log
被查询/加入的列都是必需的，无法删除，我发现很多列上没有索引，但由于它们只是单行，我不确定是否重要 - 我还能做什么吗如何加快查询速度？
答案 0 :(得分：3)
这些查询永远不会在那些硬件规范上变得那么慢。 explain输出表示所有已连接的字段使用非常优化的索引，并且仅扫描仅5566行。唯一的索引改进可能是temp_hours_worked字段week_beginning, employee_id, company_id上的综合索引，但这种情况永远不会有太大差别。根据解释输出，甚至没有任何filesorts或临时表。
我怀疑你是否遇到了锁定问题（你显示的负载很低，但是并没有告诉同一个查询在这些相同的表上运行了多少），或者你的MySQL因配置不足而功能不足（使用默认的tiny.config设置或类似设置。
要检查的事项：
hdparm -Tt /dev/sdX来测试硬盘性能 - 可能会出现SSD磁盘或RAID阵列buffer中的所有my.cnf设置至少设置为当前值的两倍，您需要备用RAM。少数可能需要更高的设置。像MySQLTuner这样的脚本可能对此有所帮助。同时检查问题是否可在其他服务器上重现。
启动MySQL缓冲区值的良好开端是将此位添加到my.cnf：
key_buffer = 768M
table_cache = 1024
sort_buffer_size = 4M
read_buffer_size = 4M
read_rnd_buffer_size = 16M
myisam_sort_buffer_size = 128M
query_cache_size = 128M
thread_concurrency = 16
table_open_cache = 2048
tmp_table_size = 64M
max_heap_table_size = 64M
您可以在phpMyAdmin（服务器 - ＆gt;变量）中查看当前值。