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本文轉載自微信公眾號「DBA閑思雜想錄」，作者瀟湘隱者 。轉載本文請聯系DBA閑思雜想錄公眾號。     

mysqldump有一個參數--lock-tables，以前對這個參數也沒有深入、詳細了解過，直到上次有個網友問“參數lock-tables 是一次性鎖定當前庫的所有表，還是鎖定當前導出表?“ ，之前一直以為只是鎖定當前導出表，后面看了參數說明后，

-l, --lock-tables   Lock all tables for read. 
 
                      (Defaults to on; use --skip-lock-tables to disable.) 

自己也不怎么確認了，當時就測試了一下。準備一個稍微大一點的庫，如果數據庫太小，那么可能mysqldum命令一下子就導出了所有庫，很難清晰的看到實驗結果。

執行下面命令做邏輯備份

[root@DB-Server ~]# mysqldump -u root -p --default-character-set=utf8  --opt --extended-insert=false --lock-tables MyDB > db_backup_MyDB.sql 
 
Enter password: 

同時立即執行下面命令

mysql> show open tables where in_use >0; 
+----------+--------------------------------+--------+-------------+ 
| Database | Table                          | In_use | Name_locked | 
+----------+--------------------------------+--------+-------------+ 
| MyDB     | AO_60DB71_VERSION              |      1 |           0 | 
| MyDB     | AO_AEFED0_TEAM_TO_MEMBER       |      1 |           0 | 
| MyDB     | AO_4B00E6_STASH_SETTINGS       |      1 |           0 | 
| MyDB     | AO_2D3BEA_FOLIOCF              |      1 |           0 | 
| MyDB     | AO_AEFED0_TEAM_ROLE            |      1 |           0 | 
| MyDB     | AO_60DB71_DETAILVIEWFIELD      |      1 |           0 | 
| MyDB     | AO_60DB71_LEXORANK             |      1 |           0 | 
| MyDB     | AO_6714C7_REPORT_SCHEDULE      |      1 |           0 | 
| MyDB     | AO_E8B6CC_SYNC_AUDIT_LOG       |      1 |           0 | 
| MyDB     | cwd_application                |      1 |           0 | 
| MyDB     | clusternode                    |      1 |           0 | 
| MyDB     | AO_86ED1B_GRACE_PERIOD         |      1 |           0 | 
| MyDB     | AO_60DB71_WORKINGDAYS          |      1 |           0 | 
| MyDB     | cwd_directory                  |      1 |           0 | 
| MyDB     | AO_2D3BEA_BASELINE             |      1 |           0 | 
| MyDB     | fieldlayoutitem                |      1 |           0 | 
| MyDB     | JQUARTZ_BLOB_TRIGGERS          |      1 |           0 | 
| MyDB     | AO_013613_HD_SCHEME_MEMBER     |      1 |           0 | 
| MyDB     | AO_2D3BEA_ALLOCATION           |      1 |           0 | 
| MyDB     | AO_013613_WL_SCHEME            |      1 |           0 | 
| MyDB     | AO_7DEABF_EXEC_CLUSTER_MESSAGE |      1 |           0 | 
.................................................................... 

如上截圖，執行mysqldump命令的時候，使用show open tables where in_use >0命令，你會看到MyDB里面的所有表的In_use的值都為1，意味著是當執行mysqldump命令時，是一次性鎖定當前庫的所有表。而不是鎖定當前導出表。

In_use說明

• The number of table locks or lock requests there are for the table. For example, if one client acquires a lock for a table using LOCK TABLE t1 WRITE, In_use will be 1. If another client issues LOCK TABLE t1 WRITE while the table remains locked, the client will block waiting for the lock, but the lock request causes In_use to be 2. If the count is zero, the table is open but not currently being used. In_use is also increased by the HANDLER ... OPEN statement and decreased by HANDLER ... CLOSE.

參考資料：

https://dev.mysql.com/doc/refman/5.7/en/show-open-tables.html

https://oracle-base.com/articles/mysql/mysql-identify-locked-tables