MySQL 8.0.32之optimize table
MySQL版本：8.0.32
最新使用到了optimize table ，不清楚是做什么用的，因此在8.0.32版本上测试了下该SQL语句。
该语句的功能是：重新组织表数据和相关索引数据的物理存储，整理碎片，减少存储空间。
关于表碎片，先看下MySQL中存储表数据时的方式，参考：MySQL表碎片是如何产生的_一缕阳光a的博客-CSDN博客
optimize table语句的功能类似于PostgreSQL中的auto vaccum。
先用一个普通表做下测试：
mysql> drop table if exists t11;
Query OK, 0 rows affected, 1 warning (0.03 sec)
mysql> create table t11(i int primary key, j varchar(200));
Query OK, 0 rows affected (0.10 sec)
mysql> insert into t11 values(1,'a'),(2,'b');
Query OK, 2 rows affected (0.02 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> create index idx1 on t11(j);
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> select count(*) from t11;
+----------+
| count(*) |
+----------+
| 2 |
+----------+
1 row in set (0.00 sec)
mysql> insert into t11 values(3,'c'),(4,'d');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select count(*) from t11;
+----------+
| count(*) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
mysql> show table status like 't11';
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
| t11 | InnoDB | 10 | Dynamic | 2 | 8192 | 16384 | 0 | 0 | 0 | NULL | 2023-04-19 15:44:32 | 2023-04-19 15:44:32 | NULL | utf8mb4_0900_ai_ci | NULL | | |
+------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+------------+--------------------+----------+----------------+---------+
1 row in set (0.00 sec)
从该表的统计信息中可以看到Rows为2，接下来执行 optimize table t11;
mysql> optimize table t11;
+----------+----------+----------+-------------------------------------------------------------------+
| Table | Op | Msg_type | Msg_text |
+----------+----------+----------+-------------------------------------------------------------------+
| test.t11 | optimize | note | Table does not support optimize, doing recreate + analyze instead |
| test.t11 | optimize | status | OK |
+----------+----------+----------+-------------------------------------------------------------------+
2 rows in set (0.12 sec)
mysql> select count(*) from t11;
+----------+
| count(*) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
mysql> show table status like 't11'\G;
*************************** 1. row ***************************
Name: t11
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 2
Avg_row_length: 8192
Data_length: 16384
Max_data_length: 0
Index_length: 0
Data_free: 0
Auto_increment: NULL
Create_time: 2023-04-19 16:11:19
Update_time: 2023-04-19 15:44:32
Check_time: NULL
Collation: utf8mb4_0900_ai_ci
Checksum: NULL
Create_options:
Comment:
1 row in set (0.01 sec)
执行 optimize table之后，查看表的统计信息显示还是ROW还是2，Index_length也是0，因此统计信息并没有修改。
进行gdb调试源码时，发现在 执行optimize table时没有 执行更新 table_stats 和 index_stats。是否更新统计信息，是由下面的ignore_grl_on_analyze变量决定的：
bool ignore_grl_on_analyze = operator_func == &handler::ha_analyze;因此只有在执行 analyze table table_name;之后才会更新统计信息。SQL语句执行如下：
mysql> analyze table t11;
+----------+---------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+----------+---------+----------+----------+
| test.t11 | analyze | status | OK |
+----------+---------+----------+----------+
1 row in set (0.02 sec)
mysql> select count(*) from t11;
+----------+
| count(*) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
mysql> show table status like 't11'\G;
*************************** 1. row ***************************
Name: t11
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 4
Avg_row_length: 4096
Data_length: 16384
Max_data_length: 0
Index_length: 16384
Data_free: 0
Auto_increment: NULL
Create_time: 2023-04-19 16:11:19
Update_time: NULL
Check_time: NULL
Collation: utf8mb4_0900_ai_ci
Checksum: NULL
Create_options:
Comment:
1 row in set (0.01 sec)
从结果可以看到，Rows: 4，Index_length: 16384，统计信息已经更到最新了。
堆栈信息如下：
mysql_execute_command
|> Sql_cmd_optimize_table::execute
||> mysql_admin_table
|||> handler::ha_optimize
||||> ha_innobase::optimize
|||> mysql_recreate_table
|||> handler::ha_analyze
||||> ha_innobase::analyze在执行 optimize table table_name; 语句时，optimize table 执行的是 recreate (mysql_recreate_table(...)) + analyze (handler::ha_analyze)，不会阻塞其他的SQL的，并且该语句是online ddl。