MySQL优化 之 Discuz论坛优化

一. 前言
近日由于需要,对discuz论坛(简称dz)进行优化,当然了,只是涉及到数据库的优化.
先说一下服务器及dz的数据量,2 * Intel(R) Xeon(TM) CPU 2.40GHz, 4GB mem, SCISC硬盘.
MySQL 版本为 4.0.23. 数据表情况:
cdb_attachments 2万
cdb_members 10万
cdb_posts 68万
cdb_threads 7万
二. 缓存优化
在 my.cnf 中添加/修改以下选项:

 #取消文件系统的外部锁
skip-locking
#不进行域名反解析,注意由此带来的权限/授权问题
skip-name-resolve
#索引缓存,根据内存大小而定,如果是独立的db服务器,可以设置高达80%的内存总量
key_buffer = 512M
#连接排队列表总数
back_log = 200
max_allowed_packet = 2M
#打开表缓存总数,可以避免频繁的打开数据表产生的开销
table_cache = 512
#每个线程排序所需的缓冲
sort_buffer_size = 4M
#每个线程读取索引所需的缓冲
read_buffer_size = 4M
#MyISAM表发生变化时重新排序所需的缓冲
myisam_sort_buffer_size = 64M
#缓存可重用的线程数
thread_cache = 128
#查询结果缓存
query_cache_size = 128M
#设置超时时间,能避免长连接
set-variable = wait_timeout=60
#最大并发线程数,cpu数量*2
thread_concurrency = 4
#记录慢查询,然后对慢查询一一优化
log-slow-queries = slow.log
long_query_time = 1
#关闭不需要的表类型,如果你需要,就不要加上这个
skip-bdb

以上参数根据各自服务器的配置差异进行调整,仅作为参考.
三. 索引优化
上面提到了,已经开启了慢查询,那么接下来就要对慢查询进行逐个优化了.
1. 搜索优化
搜索的查询SQL大致如下:

 SELECT t.* FROM cdb_posts p, cdb_threads t WHERE
t.fid IN ('37', '45', '4', '6', '17', '41', '28', '32', '31', '1', '42')
AND p.tid=t.tid AND p.author LIKE 'JoansWin'
GROUP BY t.tid ORDER BY lastpost DESC LIMIT 0, 80;

用 EXPLAIN 分析的结果如下:

 mysql>EXPLAIN  SELECT t.* FROM cdb_posts p, cdb_threads t WHERE
t.fid IN ('37', '45', '4', '6', '17', '41', '28', '32', '31', '1', '42')
AND p.tid=t.tid AND p.author LIKE 'JoansWin'
GROUP BY t.tid ORDER BY lastpost DESC LIMIT 0, 80; 
+-----------+------------+----------+--------------+-------------+-----------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref         | rows  | Extra
+-----------+------------+----------+--------------+-------------+-----------+-------------+
|  1 | SIMPLE      | t     | range | PRIMARY,fid   | fid  | 2       | NULL        | 66160 | Using where; 
Using temporary; Using filesort |
|  1 | SIMPLE      | p     | ref   | tid           | tid  | 3       | Forum.t.tid   |    10 | Using where
| +----+-------------+-------+-------+---------------+------+---------+-------------+-------+
---------

只用到了 t.fid 和 p.tid,而 p.author 则没有索引可用,总共需要扫描
66160*10 = 661600 次索引,够夸张吧 :(
再分析 cdb_threads 和 cdb_posts 的索引情况:

 mysql>show index from cdb_posts; 
+-----------+------------+----------+--------------+-------------+-----------+----------
---+----------+--------+------+--+
| Table     | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | 
Packed | Null | Index_type | Comment | +-----------+------------+----------+--------------+----
---------+-----------+-------------+----------+--------+------+--+
| cdb_posts |          0 | PRIMARY  |            1 | pid         | A         |      680114 |     NULL | NULL   |
| BTREE      |         |
| cdb_posts |          1 | fid      |            1 | fid         | A         |          10 |     NULL | NULL   |
| BTREE      |         |
| cdb_posts |          1 | tid      |            1 | tid         | A         |       68011 |     NULL | NULL   |
| BTREE      |         |
| cdb_posts |          1 | tid      |            2 | dateline    | A         |      680114 |     NULL | NULL   |
| BTREE      |         |
| cdb_posts |          1 | dateline |            1 | dateline    | A         |      680114 |     NULL | NULL   |
| BTREE      |         | 
+-----------+------------+----------+--------------+-------------+-----------+---

以及

 mysql>show index from cdb_threads; 
+-----------+------------+----------+--------------+-------------+-----------+-------------+
----------+--------+------+-----+
| Table       | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part |
Packed | Null | Index_type | Comment | +-----------+------------+----------+--------------+-----
--------+-----------+-------------+----------+--------+------+-----+
| cdb_threads |          0 | PRIMARY   |            1 | tid         | A         |       68480 |     NULL | NULL   |
| BTREE      |         |
| cdb_threads |          1 | lastpost  |            1 | topped      | A         |           4 |     NULL | NULL   |
| BTREE      |         |
| cdb_threads |          1 | lastpost  |            2 | lastpost    | A         |       68480 |     NULL | NULL   |
| BTREE      |         |
| cdb_threads |          1 | lastpost  |            3 | fid         | A         |       68480 |     NULL | NULL   |
| BTREE      |         |
| cdb_threads |          1 | replies   |            1 | replies     | A         |         233 |     NULL | NULL   |
| BTREE      |         |
| cdb_threads |          1 | dateline  |            1 | dateline    | A         |       68480 |     NULL | NULL   |
| BTREE      |         |
| cdb_threads |          1 | fid       |            1 | fid         | A         |          10 |     NULL | NULL   |
| BTREE      |         |
| cdb_threads |          1 | enablehot |            1 | enablehot   | A         |           2 |     NULL | NULL   |
| BTREE      |         | +-------------+------------+-----------+--------------+-------------+------

看到索引 fid 和 enablehot 基数太小,看来该索引完全没必要,不过,对于fid基数较大的情况,则可能需要保留>该索引.
所做修改如下:

 ALTER TABLE `cdb_threads` DROP INDEX `enablehot`, DROP INDEX `fid`, ADD INDEX (`fid`, `lastpost`);
ALTER TABLE `cdb_posts` DROP INDEX `fid`, ADD INDEX (`author`(10));
OPTIMIZE TABLE `cdb_posts`;
OPTIMIZE TABLE `cdb_threads`;

在这里, p.author 字段我设定的部分索引长度是 10, 是我经过分析后得出来的结果,不同的系统,这里的长度也不同,最好自己先取一下平均值,然后再适当调整.
现在,再来执行一次上面的慢查询,发现时间已经从 6s 变成 0.19s,提高了 30 倍.
这次先到这里,下次继续 ^_^