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- 作者： 王庆勋
- 文章起源：GreatSQL社区原创
某银行客户在从Oracle迁徙到MySQL的开发中，MySQL在READ-COMMITTED隔离级别下，呈现了insert阻塞update的状况，但同样的状况下，Oracle的insert则不会阻塞update。本文通过复现该问题，剖析MySQL的锁信息，确认是MySQL与Oracle在并发事务处理上的差别，在进行数据库迁徙革新的程序开发应予以关注。
1.问题复现
1.1.环境筹备
MySQL版本的8.0.26，隔离级别是READ-COMMITTED ，测试表t的字段a为主键。
mysql> select version();+-----------+| version() |+-----------+| 8.0.26 |+-----------+1 row in set (0.02 sec)mysql> show variables like 'transaction_isolation';+-----------------------+----------------+| Variable_name | Value |+-----------------------+----------------+| transaction_isolation | READ-COMMITTED |+-----------------------+----------------+1 row in set (0.00 sec)mysql> desc t;+-------+------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+------+------+-----+---------+-------+| a | int | NO | PRI | NULL | || b | int | YES | | NULL | |+-------+------+------+-----+---------+-------+2 rows in set (0.01 sec)mysql> select * from t;+---+------+| a | b |+---+------+| 7 | 7 |+---+------+1 row in set (0.00 sec)1.2. insert阻塞update的操作步骤
insert语句未提交时，update同样主键的数据会被阻塞。
| session1 | session2 |
|---|---|
| 插入一条数据(a=8)后未提交。 mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into t values(8,8); Query OK, 1 row affected (0.01 sec) | |
| 更改数据，条件是a=8,将会被阻塞 mysql> update t set b=0 where a=8; <<挂起，期待innodb_lock_wait_timeout超时 |
2.剖析起因
2.1.查看事务锁信息
mysql> select * from information_schema.innodb_trx\G*************************** 1. row *************************** trx_id: 3795 trx_state: LOCK WAIT trx_started: 2022-10-11 16:03:38 trx_requested_lock_id: 139727275779216:52:4:3:139724882995456 trx_wait_started: 2022-10-11 16:03:38 trx_weight: 2 trx_mysql_thread_id: 9346 trx_query: update t set b=0 where a=8 trx_operation_state: starting index read trx_tables_in_use: 1 trx_tables_locked: 1 trx_lock_structs: 2 trx_lock_memory_bytes: 1128 trx_rows_locked: 1 trx_rows_modified: 0 trx_concurrency_tickets: 0 trx_isolation_level: READ COMMITTED trx_unique_checks: 1 trx_foreign_key_checks: 1trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0 trx_is_read_only: 0trx_autocommit_non_locking: 0 trx_schedule_weight: 1*************************** 2. row *************************** trx_id: 3790 trx_state: RUNNING trx_started: 2022-10-11 16:03:29 trx_requested_lock_id: NULL trx_wait_started: NULL trx_weight: 3 trx_mysql_thread_id: 9320 trx_query: NULL trx_operation_state: NULL trx_tables_in_use: 0 trx_tables_locked: 1 trx_lock_structs: 2 trx_lock_memory_bytes: 1128 trx_rows_locked: 1 trx_rows_modified: 1 trx_concurrency_tickets: 0 trx_isolation_level: READ COMMITTED trx_unique_checks: 1 trx_foreign_key_checks: 1trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0 trx_is_read_only: 0trx_autocommit_non_locking: 0 trx_schedule_weight: NULL2 rows in set (0.00 sec)阐明：通过InnoDB的事务表innodb_trx查问到thread_id=9346的事务3795正在期待锁（trx_state: LOCK WAIT）,thread_id=9320的事务3790正在执行（trx_state: RUNNING）。mysql> select * from performance_schema.data_locks\G*************************** 1. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 139727275779216:1113:139724882998560ENGINE_TRANSACTION_ID: 3795 THREAD_ID: 9441 EVENT_ID: 5000 OBJECT_SCHEMA: testdb OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: NULLOBJECT_INSTANCE_BEGIN: 139724882998560 LOCK_TYPE: TABLE LOCK_MODE: IX LOCK_STATUS: GRANTED LOCK_DATA: NULL*************************** 2. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 139727275779216:52:4:3:139724882995456ENGINE_TRANSACTION_ID: 3795 THREAD_ID: 9441 EVENT_ID: 5012 OBJECT_SCHEMA: testdb OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: PRIMARYOBJECT_INSTANCE_BEGIN: 139724882995456 LOCK_TYPE: RECORD LOCK_MODE: X,REC_NOT_GAP LOCK_STATUS: WAITING LOCK_DATA: 8*************************** 3. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 139727275781640:1113:139724883017072ENGINE_TRANSACTION_ID: 3790 THREAD_ID: 9415 EVENT_ID: 15467 OBJECT_SCHEMA: testdb OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: NULLOBJECT_INSTANCE_BEGIN: 139724883017072 LOCK_TYPE: TABLE LOCK_MODE: IX LOCK_STATUS: GRANTED LOCK_DATA: NULL*************************** 4. row *************************** ENGINE: INNODB ENGINE_LOCK_ID: 139727275781640:52:4:3:139724883013968ENGINE_TRANSACTION_ID: 3790 THREAD_ID: 9441 EVENT_ID: 5007 OBJECT_SCHEMA: testdb OBJECT_NAME: t PARTITION_NAME: NULL SUBPARTITION_NAME: NULL INDEX_NAME: PRIMARYOBJECT_INSTANCE_BEGIN: 139724883013968 LOCK_TYPE: RECORD LOCK_MODE: X,REC_NOT_GAP LOCK_STATUS: GRANTED LOCK_DATA: 84 rows in set (0.00 sec)阐明：事务3795正在期待LOCK_TYPE: RECORD，LOCK_MODE:X,REC_NOT_GAP，期待的主键值为8；事务3790已获取主键值为8的LOCK_TYPE: RECORD，LOCK_MODE:X,REC_NOT_GAP锁。mysql> select * from sys.innodb_lock_waits\G*************************** 1. row *************************** wait_started: 2022-10-11 16:03:38 wait_age: 00:02:50 wait_age_secs: 170 locked_table: `testdb`.`t` locked_table_schema: testdb locked_table_name: t locked_table_partition: NULL locked_table_subpartition: NULL locked_index: PRIMARY locked_type: RECORD waiting_trx_id: 3795 waiting_trx_started: 2022-10-11 16:03:38 waiting_trx_age: 00:02:50 waiting_trx_rows_locked: 1 waiting_trx_rows_modified: 0 waiting_pid: 9346 waiting_query: update t set b=0 where a=8 waiting_lock_id: 139727275779216:52:4:3:139724882995456 waiting_lock_mode: X,REC_NOT_GAP blocking_trx_id: 3790 blocking_pid: 9320 blocking_query: NULL blocking_lock_id: 139727275781640:52:4:3:139724883013968 blocking_lock_mode: X,REC_NOT_GAP blocking_trx_started: 2022-10-11 16:03:29 blocking_trx_age: 00:02:59 blocking_trx_rows_locked: 1 blocking_trx_rows_modified: 1 sql_kill_blocking_query: KILL QUERY 9320sql_kill_blocking_connection: KILL 93201 row in set (0.01 sec)阐明：事务3795期待testdb.t上的rec_not_gap独占锁，事务3790持有该独占锁。mysql> select distinct,* from sys.processlist where conn_id in (select trx_mysql_thread_id from information_schema.innodb_trx)\G*************************** 1. row *************************** thd_id: 9441 conn_id: 9346 user: admin@172.17.128.73 db: testdb command: Query state: updating time: 141 current_statement: update t set b=0 where a=8 statement_latency: 2.37 min progress: NULL lock_latency: 431.00 us rows_examined: 0 rows_sent: 0 rows_affected: 0 tmp_tables: 0 tmp_disk_tables: 0 full_scan: NO last_statement: NULLlast_statement_latency: NULL current_memory: 140.15 KiB last_wait: wait/io/table/sql/handler last_wait_latency: Still Waiting source: handler.cc:3250 trx_latency: 13.30 min trx_state: ACTIVE trx_autocommit: NO pid: 9632 program_name: mysql*************************** 2. row *************************** thd_id: 9415 conn_id: 9320 user: admin@172.17.128.73 db: testdb command: Sleep state: NULL time: 801 current_statement: NULL statement_latency: NULL progress: NULL lock_latency: 288.00 us rows_examined: 0 rows_sent: 0 rows_affected: 1 tmp_tables: 0 tmp_disk_tables: 0 full_scan: NO last_statement: insert into t values(8,8)last_statement_latency: 765.23 us current_memory: 218.19 KiB last_wait: wait/io/socket/sql/client_connection last_wait_latency: Still Waiting source: viosocket.cc:146 trx_latency: 13.52 min trx_state: ACTIVE trx_autocommit: NO pid: 9600 program_name: mysql阐明：被阻塞事务执行的sql语句update t set b=0 where a=8，阻塞事务执行的sql语句是insert into t values(8,8)。阐明：
MySQL的隔离级别是通过索引上的锁实现并发事务管制的。在READ-COMMITTED隔离级别下，session1在执行insert语句时，在主键索引上获取了a=8的行记录独占锁，以禁止插入雷同主键的数据；session2如果同时插入雷同的主键数据被阻塞，容易了解（Oracle也同样阻塞）。出于同样的起因session2执行update时，因为无奈获取a=8的行记录独占锁，同样也会被阻塞。
2.2.验证MySQL事务未提交时已写入数据文件
验证事务未提交时，insert语句已将数据写入数据文件，索引数据也已生成。
测试表test1mysql> CREATE TABLE `test1` ( -> `id` int NOT NULL AUTO_INCREMENT, -> `k` int NOT NULL DEFAULT '0', -> `c` char(120) NOT NULL DEFAULT '', -> `pad` char(60) NOT NULL DEFAULT '', -> PRIMARY KEY (`id`), -> KEY `k_1` (`k`) -> ) ;Query OK, 0 rows affected (0.07 sec)开启一个事务，插入10万条数据。mysql> begin;Query OK, 0 rows affected (0.00 sec)mysql> insert into test1 select * from sbtest1;Query OK, 100000 rows affected (1.44 sec)Records: 100000 Duplicates: 0 Warnings: 0检查表的data_length和index_lengthmysql> show table status where name like 'test1'\G*************************** 1. row *************************** Name: test1 Engine: InnoDB Version: 10 Row_format: Dynamic Rows: 98712 Avg_row_length: 228 Data_length: 22593536Max_data_length: 0 Index_length: 2637824 Data_free: 4194304 Auto_increment: 100001 Create_time: 2022-10-11 22:14:50 Update_time: NULL Check_time: NULL Collation: utf8mb4_0900_ai_ci Checksum: NULL Create_options: Comment: 1 rows in set (0.01 sec)回滚insert操作mysql> rollback;Query OK, 0 rows affected (1.35 sec)更新统计信息mysql> analyze table test1;再次检查表的data_length和index_lengthmysql> show table status where name like 'test1'\G *************************** 1. row *************************** Name: test1 Engine: InnoDB Version: 10 Row_format: Dynamic Rows: 0 Avg_row_length: 0 Data_length: 16384Max_data_length: 0 Index_length: 16384 Data_free: 29360128 Auto_increment: 100001 Create_time: 2022-10-11 22:22:36 Update_time: NULL Check_time: NULL Collation: utf8mb4_0900_ai_ci Checksum: NULL Create_options: Comment: 1 row in set (0.00 sec)查看数据文件的大小[root@host73 testdb]# ll *test1.ibd-rw-r----- 1 greatdb greatdb 30408704 Oct 11 15:12 sbtest1.ibd-rw-r----- 1 greatdb greatdb 33554432 Oct 11 22:24 test1.ibd阐明：
MySQL在执行insert 语句进行数据插入，未提交时，数据也已写入表的汇集索引，辅助索引也已生成。MySQL能够应用未提交数据的索引，通过锁机制实现事务的并发管制。
3.Oracle中insert没有阻塞update
在Oracle中，创立同样的测试表t，执行同样的insert和update，但insert不会阻塞update。
CREATE TABLE t ( a int NOT NULL PRIMARY KEY , b int DEFAULT NULL);insert into t values(7,7);commit;执行雷同的insert和update语句。
| session1 | session2 |
|---|---|
| SQL> insert into t values(8,8); | |
| 1 row created. | SQL> update t set b=0 where a=8;0 rows updated. |
Enjoy GreatSQL :)
## 对于 GreatSQL
GreatSQL是由万里数据库保护的MySQL分支，专一于晋升MGR可靠性及性能，反对InnoDB并行查问个性，是实用于金融级利用的MySQL分支版本。
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