为什么 'lock_type' of information_schema.INNODB_LOCKS 总是 RECORD？

Question

根据文档：https://dev.mysql.com/doc/refman/5.7/en/innodb-locks-table.html、

LOCK_TYPE of INFORMATION_SCHEMA INNODB_LOCKS Table should have 2 possible values: RECORD AND TABLE.

但是，我还没有看到那个字段是 "TABLE"。无论我尝试什么，它总是 "RECORD"。有没有人可以给我一个案例来实现 "TABLE"？

谢谢，叶先义

Answer 1

what Mysql version are you using?

Mysql8 INFORMATION SCHEMA does not have the table INNODB_LOCKS
mysql> show tables like '%innodb%';
+-----------------------------------------+
| Tables_in_information_schema (%INNODB%) |
+-----------------------------------------+
| INNODB_BUFFER_PAGE                      |
| INNODB_BUFFER_PAGE_LRU                  |
| INNODB_BUFFER_POOL_STATS                |
| INNODB_CACHED_INDEXES                   |
| INNODB_CMP                              |
| INNODB_CMPMEM                           |
| INNODB_CMPMEM_RESET                     |
| INNODB_CMP_PER_INDEX                    |
| INNODB_CMP_PER_INDEX_RESET              |
| INNODB_CMP_RESET                        |
| INNODB_COLUMNS                          |
| INNODB_DATAFILES                        |
| INNODB_FIELDS                           |
| INNODB_FOREIGN                          |
| INNODB_FOREIGN_COLS                     |
| INNODB_FT_BEING_DELETED                 |
| INNODB_FT_CONFIG                        |
| INNODB_FT_DEFAULT_STOPWORD              |
| INNODB_FT_DELETED                       |
| INNODB_FT_INDEX_CACHE                   |
| INNODB_FT_INDEX_TABLE                   |
| INNODB_INDEXES                          |
| INNODB_METRICS                          |
| INNODB_TABLES                           |
| INNODB_TABLESPACES                      |
| INNODB_TABLESPACES_BRIEF                |
| INNODB_TABLESTATS                       |
| INNODB_TEMP_TABLE_INFO                  |
| INNODB_TRX                              |
| INNODB_VIRTUAL                          |
+-----------------------------------------+
30 rows in set (0.01 sec)

参考 http://drib.tech/programming/debug-monitor-mysql-locks

Answer 2

您 link 阅读了一些 MySQL 5.7 文档，所以我假设您使用的是 5.7。

InnoDB 中唯一的 table 锁是 元数据锁 ，也称为 意向锁 ，在此处记录：https://dev.mysql.com/doc/refman/5.5/en/innodb-locking.html#innodb-intention-locks

任何想要执行某些 DML 操作以读取或写入 table（即 SELECT/INSERT/UPDATE/DELETE）中的记录的事务都需要先获取元数据锁。多个并发会话可以获取元数据锁来做DML，并且这些不会相互冲突。

同样，元数据操作（DDL 语句 ALTER、DROP 或 TRUNCATE TABLE）在开始之前需要获取元数据锁。因此，如果存在任何事务运行 DML 语句和 vice-versa，则 DDL 语句将被阻止。

这就是在 long-running ALTER 期间阻止 DML 查询 table 的方式。

在 MySQL 5.7 中，您可以为元数据锁设置一些 PERFORMANCE_SCHEMA 工具，但这需要相当多的工作。参见 https://www.percona.com/blog/2016/12/28/quickly-troubleshooting-metadata-locks-mysql-5-7/

在 MySQL 8.0.1 中，默认情况下有一个新的 PERFORMANCE_SCHEMA.DATA_LOCKS 来监视这些锁。这是一个例子：

mysql> select * from performance_schema.data_locks\G
*************************** 1. row ***************************
               ENGINE: INNODB
       ENGINE_LOCK_ID: 4874033848:1112:140194859105464
ENGINE_TRANSACTION_ID: 15171
            THREAD_ID: 49
             EVENT_ID: 12
        OBJECT_SCHEMA: test
          OBJECT_NAME: mytable
       PARTITION_NAME: NULL
    SUBPARTITION_NAME: NULL
           INDEX_NAME: NULL
OBJECT_INSTANCE_BEGIN: 140194859105464
            LOCK_TYPE: TABLE                <-- TABLE lock
            LOCK_MODE: IX                   <-- "IX" intention lock mode
          LOCK_STATUS: GRANTED
            LOCK_DATA: NULL
*************************** 2. row ***************************
               ENGINE: INNODB
       ENGINE_LOCK_ID: 4874033848:55:4:2:140194863232024
ENGINE_TRANSACTION_ID: 15171
            THREAD_ID: 49
             EVENT_ID: 12
        OBJECT_SCHEMA: test
          OBJECT_NAME: mytable
       PARTITION_NAME: NULL
    SUBPARTITION_NAME: NULL
           INDEX_NAME: PRIMARY
OBJECT_INSTANCE_BEGIN: 140194863232024
            LOCK_TYPE: RECORD
            LOCK_MODE: X
          LOCK_STATUS: WAITING
            LOCK_DATA: 1

还有一个METADATA_LOCKS table:

mysql> select * from performance_schema.metadata_locks;
+-------------+--------------------+----------------+-------------+-----------------------+---------------------+---------------+-------------+-------------------+-----------------+----------------+
| OBJECT_TYPE | OBJECT_SCHEMA      | OBJECT_NAME    | COLUMN_NAME | OBJECT_INSTANCE_BEGIN | LOCK_TYPE           | LOCK_DURATION | LOCK_STATUS | SOURCE            | OWNER_THREAD_ID | OWNER_EVENT_ID |
+-------------+--------------------+----------------+-------------+-----------------------+---------------------+---------------+-------------+-------------------+-----------------+----------------+
| TABLE       | test               | mytable        | NULL        |       140194860588144 | SHARED_WRITE        | TRANSACTION   | GRANTED     | sql_parse.cc:5820 |              48 |             26 |
...

为什么 'lock_type' of information_schema.INNODB_LOCKS 总是 RECORD？

Why does 'lock_type' of information_schema.INNODB_LOCKS always RECORD?

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