does the optimize table of mysql really rebuild(recreate) table？ - mysql

Question

I have a question about optimize table of mysql,I have read many documents about this function,all of the document says：it can defragment an InnoDB table and reorganizes the physical storage of table data and associated index data, to reduce storage space,but no one can tell me the principle of optimize table and slove my questions,I have known those informations:
As of MySQL 5.6.17,OPTIMIZE TABLE uses online DDL for regular and partitioned InnoDB tables
,As of MySQL 5.6.17,The table rebuild triggered by OPTIMIZE TABLE and performed under the cover by ALTER TABLE ... FORCE is completed in place(ALGORITHM =inplace)
my questions:
does the optimize table of mysql really rebuild(recreate) table？ because it completed in place(not ALGORITHM=copy),is ALGORITHM=inplace real rebuild(recreate) table?
thank you for your response.
https://dev.mysql.com/doc/refman/5.6/en/innodb-online-ddl-operations.html
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Answer 1

Demo:
I created a table on MySQL 5.6.37 and filled it with about 1GB of random data.
mysql> select version();
+-----------------+
| version() |
+-----------------+
| 5.6.37-82.2-log |
+-----------------+
mysql> show table status\G
*************************** 1. row ***************************
Name: mytable
Engine: InnoDB
Version: 10
Row_format: Compact
Rows: 61914
Avg_row_length: 20926
Data_length: 1295613952
Max_data_length: 0
Index_length: 1589248
Data_free: 4194304
Auto_increment: 131056
Create_time: 2019-02-20 15:54:23
Update_time: NULL
Check_time: NULL
Collation: utf8mb4_general_ci
Checksum: NULL
Create_options:
Comment:
I can observe the file in my datadir and note the inode number (the leftmost number).
/usr/local/var/mysql/test ls -li
total 2590752
8625784850 -rw-rw---- 1 bkarwin admin 67 Feb 20 07:54 db.opt
8625784853 -rw-rw---- 1 bkarwin admin 8580 Feb 20 07:54 mytable.frm
8625784854 -rw-rw---- 1 bkarwin admin 1321205760 Feb 20 07:56 mytable.ibd
I run optimize table.
mysql> optimize table mytable;
While it's running, I check progress by listing files in the datadir again. I see it has created a new temp file and it's filling it with a copy of the data.
Note the inode number of the temp file.
/usr/local/var/mysql/test ls -li
total 4395064
8625785082 -rw-rw---- 1 bkarwin admin 8580 Feb 20 07:57 #sql-337_16.frm
8625785083 -rw-rw---- 1 bkarwin admin 914358272 Feb 20 07:57 #sql-ib2021-3689447301.ibd
8625784850 -rw-rw---- 1 bkarwin admin 67 Feb 20 07:54 db.opt
8625784853 -rw-rw---- 1 bkarwin admin 8580 Feb 20 07:54 mytable.frm
8625784854 -rw-rw---- 1 bkarwin admin 1321205760 Feb 20 07:56 mytable.ibd
Finally my optimize table finishes. It took almost 2 minutes. If it wasn't doing a real table copy, why would it have taken so long?
+--------------+----------+----------+-------------------------------------------------------------------+
| Table | Op | Msg_type | Msg_text |
+--------------+----------+----------+-------------------------------------------------------------------+
| test.mytable | optimize | note | Table does not support optimize, doing recreate + analyze instead |
| test.mytable | optimize | status | OK |
+--------------+----------+----------+-------------------------------------------------------------------+
2 rows in set (1 min 5.52 sec)
Looking at the datadir again, I see the temp file has been renamed to be the new tablespace, and the old file has been removed. The new file has the inode number of the temp file, indicating it's the copy.
/usr/local/var/mysql/test ls -li
total 2557984
8625784850 -rw-rw---- 1 bkarwin admin 67 Feb 20 07:54 db.opt
8625785082 -rw-rw---- 1 bkarwin admin 8580 Feb 20 07:57 mytable.frm
8625785083 -rw-rw---- 1 bkarwin admin 1304428544 Feb 20 07:58 mytable.ibd