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回收InnoDB表空间
以下论述均假定innodb_file_per_table开启
先用常规optimize回收：
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mysql> select count(*) from t; <p>+----------+ </p><p>| count(*) | </p><p>+----------+ </p><p>| 1200096 | </p><p>+----------+ </p><p> </p><p>[mysql@even employees]$ ls -alh t.ibd </p><p>-rw-rw---- 1 mysql dba 72M 10-08 17:39 t.ibd </p><p> </p><p>mysql> delete from t limit 800000; </p><p> </p><p>mysql> select count(*) from t; </p><p>+----------+ </p><p>| count(*) | </p><p>+----------+ </p><p>| 400096 | </p><p>+----------+ </p><p> </p><p>[mysql@even employees]$ ls -alh t.ibd </p><p>-rw-rw---- 1 mysql dba 72M 10-08 17:41 t.ibd </p><p> </p><p> </p><p>mysql> optimize table t; </p><p>+-------------+----------+----------+-------------------------------------------------------------------+ </p><p>| Table | Op | Msg_type | Msg_text | </p><p>+-------------+----------+----------+-------------------------------------------------------------------+ </p><p>| employees.t | optimize | note | Table does not support optimize, doing recreate + analyze instead | </p><p>| employees.t | optimize | status | OK | </p><p>+-------------+----------+----------+-------------------------------------------------------------------+ </p><p> </p><p>[mysql@even employees]$ ls -alh t.ibd </p><p>-rw-rw---- 1 mysql dba 29M 10-08 17:43 t.ibd</p>
来看下这种方法的缺陷
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mysql> show processlist;
+----+------+-----------+-----------+---------+------+-------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+-----------+---------+------+-------------------+------------------+
| 5 | root | localhost | NULL | Query | 0 | NULL | show processlist |
| 6 | root | localhost | employees | Query | 64 | copy to tmp table | optimize table t |
+----+------+-----------+-----------+---------+------+-------------------+------------------+
mysqladmin debug结果如下
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Thread database.table_name Locked/Waiting Lock_type
6 employees.t Locked - read Read lock without concurrent inserts
用optimize缺点显而易见：读锁，特别当你有大表时性能恐怕会深受影响
这里推荐使用percona公司：pt-online-schema-change，避免加锁
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$ pt-online-schema-change -uroot -poracle --alter "ENGINE=InnoDB" D=employees,t=t --execute
执行pt工具时有可能发生的类似错误：
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Cannot chunk the original table There is no good index and the table is oversized
这是因为被作用的表需要含有主键或者唯一索引，这或许也能成为这款工具的小bug吧
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