mysql中OPTIMIZE TABLE的作用

1、先来看看多次删除插入操作后的表索引情况

mysql> SHOW INDEX FROM `tbl_name`;
+----------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| tbl_name | 0 | PRIMARY | 1 | StepID | A | 1 | NULL | NULL | | BTREE | |
| tbl_name | 1 | FlowID | 1 | FlowID | A | 1 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerCount | 1 | WagerCount | A | 1 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_3 | 1 | WagerID | A | 1 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_3 | 2 | StepType | A | 1 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_3 | 3 | ParamResult | A | 1 | 255 | NULL | | BTREE | |
| tbl_name | 1 | StepType_2 | 1 | StepType | A | 1 | NULL | NULL | | BTREE | |
| tbl_name | 1 | StepType_2 | 2 | ParamResult | A | 1 | 255 | NULL | | BTREE | |
| tbl_name | 1 | WagerID_2 | 1 | WagerID | A | 1 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_2 | 2 | StepType | A | 1 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_2 | 3 | ParamResult | A | 1 | 255 | NULL | | BTREE | |
+----------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
11 rows in set (0.01 sec)

2、优化表

mysql> optimize table tbl_name;
+---------------+----------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+---------------+----------+----------+----------+
| test.tbl_name | optimize | status | OK |
+---------------+----------+----------+----------+
1 row in set (40.60 sec)

3、再来看看优化后的效果

mysql> SHOW INDEX FROM `tbl_name`;
+----------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| tbl_name | 0 | PRIMARY | 1 | StepID | A | 172462 | NULL | NULL | | BTREE | |
| tbl_name | 1 | FlowID | 1 | FlowID | A | 86231 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerCount | 1 | WagerCount | A | 4311 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_3 | 1 | WagerID | A | 86231 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_3 | 2 | StepType | A | 172462 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_3 | 3 | ParamResult | A | 172462 | 255 | NULL | | BTREE | |
| tbl_name | 1 | StepType_2 | 1 | StepType | A | 9 | NULL | NULL | | BTREE | |
| tbl_name | 1 | StepType_2 | 2 | ParamResult | A | 86231 | 255 | NULL | | BTREE | |
| tbl_name | 1 | WagerID_2 | 1 | WagerID | A | 86231 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_2 | 2 | StepType | A | 172462 | NULL | NULL | | BTREE | |
| tbl_name | 1 | WagerID_2 | 3 | ParamResult | A | 172462 | 255 | NULL | | BTREE | |
+----------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

最后，来看看手册中关于 OPTIMIZE 的描述：

OPTIMIZE [LOCAL | NO_WRITE_TO_BINLOG] TABLE tbl_name [, tbl_name] ...

如果您已经删除了表的一大部分，或者如果您已经对含有可变长度行的表（含有VARCHAR, BLOB或TEXT列的表）进行了很多更改，则应使用
OPTIMIZE TABLE。被删除的记录被保持在链接清单中，后续的INSERT操作会重新使用旧的记录位置。您可以使用OPTIMIZE TABLE来重新
利用未使用的空间，并整理数据文件的碎片。

在多数的设置中，您根本不需要运行OPTIMIZE TABLE。即使您对可变长度的行进行了大量的更新，您也不需要经常运行，每周一次或每月一次
即可，只对特定的表运行。

OPTIMIZE TABLE只对MyISAM, BDB和InnoDB表起作用。

注意，在OPTIMIZE TABLE运行过程中，MySQL会锁定表。