MySQL針對Discuz論壇程序的基本優化教程

過了這麼久，discuz論壇的問題還是困擾著很多網友，其實從各論壇裡看到的問題總結出來，很關鍵的一點都是因為沒有將數據表引擎轉成InnoDB導致的，discuz在並發稍微高一點的環境下就表現的非常糟糕，產生大量的鎖等待，這時候如果把數據表引擎改成InnoDB的話，我相信會好很多。這次就寫個掃盲貼吧。

1. 啟用innodb引擎，並配置相關參數

#skip-innodb

innodb_additional_mem_pool_size = 16M #一般16M也夠了，可以適當調整下
innodb_buffer_pool_size = 6G #如果是專用db的話，一般是內存總量的80%
innodb_data_file_path = ibdata1:1024M:autoextend
innodb_file_io_threads = 4
innodb_thread_concurrency = 20
innodb_flush_log_at_trx_commit = 1
innodb_log_buffer_size = 16M
innodb_log_file_size = 256M
innodb_log_files_in_group = 3
innodb_max_dirty_pages_pct = 50
innodb_lock_wait_timeout = 120
innodb_file_per_table

修改表引擎為innodb：

mysql> alter table cdb_access engine = innodb;

其他表類似上面，把表名換一下即可...
將表存儲引擎改成innodb後，不僅可以避免大量的鎖等待，還可以提升查詢的效率，因為innodb會把data和index都放在buffer pool中，效率更高。

2.緩存優化
在 my.cnf 中添加/修改以下選項:

 #取消文件系統的外部鎖
skip-locking
#不進行域名反解析,注意由此帶來的權限/授權問題
skip-name-resolve
#索引緩存,根據內存大小而定,如果是獨立的db服務器,可以設置高達80%的內存總量
key_buffer = 512M
#連接排隊列表總數
back_log = 200
max_allowed_packet = 2M
#打開表緩存總數,可以避免頻繁的打開數據表產生的開銷
table_cache = 512
#每個線程排序所需的緩沖
sort_buffer_size = 4M
#每個線程讀取索引所需的緩沖
read_buffer_size = 4M
#MyISAM表發生變化時重新排序所需的緩沖
myisam_sort_buffer_size = 64M
#緩存可重用的線程數
thread_cache = 128
#查詢結果緩存
query_cache_size = 128M
#設置超時時間,能避免長連接
set-variable = wait_timeout=60
#最大並發線程數,cpu數量*2
thread_concurrency = 4
#記錄慢查詢,然後對慢查詢一一優化
log-slow-queries = slow.log
long_query_time = 1
#關閉不需要的表類型,如果你需要,就不要加上這個
skip-bdb

以上參數根據各自服務器的配置差異進行調整,僅作為參考.

3.索引優化
上面提到了,已經開啟了慢查詢,那麼接下來就要對慢查詢進行逐個優化了.

搜索的查詢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 分析的結果如下:

 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 則沒有索引可用,總共需要掃描
66160*10 = 661600 次索引,夠誇張吧 :(
再分析 cdb_threads 和 cdb_posts 的索引情況:

 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 基數太小,看來該索引完全沒必要,不過,對於fid基數較大的情況,則可能需要保留>該索引.
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`;

在這裡, p.author 字段我設定的部分索引長度是 10, 是我經過分析後得出來的結果,不同的系統,這裡的長度也不同,最好自己先取一下平均值,然後再適當調整.
現在,再來執行一次上面的慢查詢,發現時間已經從 6s 變成 0.19s,提高了 30 倍.