原文地址: http://www.cnblogs.com/zaric/archive/2012/09/28/2707248.html
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今天優化了多條SQL語句。都是EXPLAIN的功勞,分析SQL子句的執行順序和執行情況,一木瞭然,下來看具體分析:
[優化多表聯合查詢]
explain SELECT sql_no_cache pker.*,pk.* FROM ng_game_pk AS pk ,ng_game_pker AS pker where pker.pkid = pk.id and (pker.act_uid = 1 OR pker.def_uid = 1) AND pk.type <>4 GROUP BY pk.id limit 10; +----+-------------+-------+--------+----------------------------------+---------+---------+--------------------+--------+----------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+----------------------------------+---------+---------+--------------------+--------+----------------------------------------------+ | 1 | SIMPLE | pker | ALL | pkid,act_def | NULL | NULL | NULL | 177543 | Using where; Using temporary; Using filesort | | 1 | SIMPLE | pk | eq_ref | PRIMARY,type,idx_type_status_uid | PRIMARY | 4 | pwgbk8.7.pker.pkid | 1 | Using where | +----+-------------+-------+--------+----------------------------------+---------+---------+--------------------+--------+----------------------------------------------+
key:NULL,,,Extra:Using temporary,Using filesort都會造成語句很慢。type:ALL 全表掃描,沒有比這個更糟糕的了。
下面作出改動:
explain SELECT sql_no_cache pker.*,pk.* FROM ng_game_pk AS pk ,ng_game_pker AS pker where pker.pkid = pk.id and (pker.act_uid = 1 OR pker.def_uid = 1) AND pk.type <>4 GROUP BY pker.pkid limit 10; +----+-------------+-------+--------+----------------------------------+---------+---------+--------------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+----------------------------------+---------+---------+--------------------+------+-------------+ | 1 | SIMPLE | pker | index | pkid,act_def | pkid | 4 | NULL | 10 | Using where | | 1 | SIMPLE | pk | eq_ref | PRIMARY,type,idx_type_status_uid | PRIMARY | 4 | pwgbk8.7.pker.pkid | 1 | Using where | +----+-------------+-------+--------+----------------------------------+---------+---------+--------------------+------+-------------+
比較下執行:
mysql> show profile for query 147; +----------------------+----------+ | Status | Duration | +----------------------+----------+ | Creating tmp table | 0.000372 | | Copying to tmp table | 0.424248 | | removing tmp table | 0.002125 | +----------------------+----------+
這裏忽略了其他的執行。看到拷貝到臨時表佔用了大部分時間。這裏需要解釋下:Copying to tmp table是拷貝到內存,如果是Copying to tmp table on disk意味着內存空間不足,MySQL將會寫臨時表到磁盤。這個大小的配置參考tmp_table_size。
現分析下第二條SQL,,,子句id是一樣的,那麼執行順序是從上至下,現查詢pker表,索引爲pkid,那麼可以認爲GROUP BY pker.pkid作用於上面的子句,rows爲10,那麼limit 10也是作用於此。
[優化索引、複合索引]
mysql> select count(*) from ng_game_pk WHERE status = 0 AND type = 4 AND uid = 1; +----------+ | count(*) | +----------+ | 0 | +----------+ 1 row in set (13.03 sec)
mysql> explain select count(*) from ng_game_pk WHERE status = 0 AND type = 4 AND uid = 1;
+----+-------------+------------+------+---------------+------+---------+-------+---------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+---------------+------+---------+-------+---------+-------------+
| 1 | SIMPLE | ng_game_pk | ref | type | type | 1 | const | 1729551 | Using where |
+----+-------------+------------+------+---------------+------+---------+-------+---------+-------------
這條語句用到了索引type,但是type取值範圍很窄(1,2,3,4)其實這個索引沒多大用處。
下面我們建立複合索引看效果如何,
mysql> alter table ng_game_pk add index idx_type_status_uid(type,status,uid); Query OK, 5831851 rows affected (1 min 43.20 sec) Records: 5831851 Duplicates: 0 Warnings: 0 mysql> explain select count(*) from ng_game_pk WHERE status = 0 AND type = 4 AND uid = 1; +----+-------------+------------+------+--------------------------+---------------------+---------+-------------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+--------------------------+---------------------+---------+-------------------+------+-------------+ | 1 | SIMPLE | ng_game_pk | ref | type,idx_type_status_uid | idx_type_status_uid | 6 | const,const,const | 1 | Using index | +----+-------------+------------+------+--------------------------+---------------------+---------+-------------------+------+-------------+ 1 row in set (0.11 sec) mysql> select count(*) from ng_game_pk WHERE status = 0 AND type = 4 AND uid = 1; +----------+ | count(*) | +----------+ | 0 | +----------+ 1 row in set (0.00 sec)
看來有很好的效果了。對SQL稍作修改,
mysql> select sql_no_cache count(*) from ng_game_pk WHERE type = 4 and status>0 AND uid = 1; +----------+ | count(*) | +----------+ | 2649 | +----------+ 1 row in set (0.40 sec)
性能又下來了,這是因爲B-Tree算法的原因,存儲引擎將不能優化任何在第一個條件範圍右邊的列。那麼就是(type,status,,,)後面的索引失效。
那麼調整下,,,
mysql> drop index idx_type_status_uid on ng_game_pk; mysql> alter table ng_game_pk add index idx_type_uid_status (type,uid,status);
結果性能又提升上來了。
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參考:
#《高性能MySql (第二版)》
#《構建高性能web站點》
# http://www.cnitblog.com/aliyiyi08/archive/2008/09/09/48878.html
# http://www.perfgeeks.com/?p=460