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不同于Oracle:MySQL的insert会阻塞update

王庆勋 GreatSQL社区 2024-07-08

* GreatSQL使

某银行客户在从Oracle迁移到MySQL的开发中,MySQL在READ-COMMITTED隔离级别下,出现了insert阻塞update的情况,但同样的情况下,Oracle的insert则不会阻塞update。本文通过复现该问题,分析MySQL的锁信息,确认是MySQL与Oracle在并发事务处理上的差异,在进行数据库迁移改造的程序开发应予以关注。

1.问题复现

1.1.环境准备

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。

session1session2
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,等待innodb_lock_wait_timeout超时

2.分析原因

2.1.检查事务锁信息

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: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 0
trx_is_read_only: 0
trx_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: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 0
trx_is_read_only: 0
trx_autocommit_non_locking: 0
trx_schedule_weight: NULL
2 rows in set (0.00 sec)

说明:通过InnoDB的事务表innodb_trx查询到thread_id=9346的事务3795正在等待锁(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: 139727275779216:1113:139724882998560
ENGINE_TRANSACTION_ID: 3795
THREAD_ID: 9441
EVENT_ID: 5000
OBJECT_SCHEMA: testdb
OBJECT_NAME: t
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: NULL
OBJECT_INSTANCE_BEGIN: 139724882998560
LOCK_TYPE: TABLE
LOCK_MODE: IX
LOCK_STATUS: GRANTED
LOCK_DATA: NULL
*************************** 2. row ***************************
ENGINE: INNODB
ENGINE_LOCK_ID: 139727275779216:52:4:3:139724882995456
ENGINE_TRANSACTION_ID: 3795
THREAD_ID: 9441
EVENT_ID: 5012
OBJECT_SCHEMA: testdb
OBJECT_NAME: t
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: PRIMARY
OBJECT_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: 139727275781640:1113:139724883017072
ENGINE_TRANSACTION_ID: 3790
THREAD_ID: 9415
EVENT_ID: 15467
OBJECT_SCHEMA: testdb
OBJECT_NAME: t
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: NULL
OBJECT_INSTANCE_BEGIN: 139724883017072
LOCK_TYPE: TABLE
LOCK_MODE: IX
LOCK_STATUS: GRANTED
LOCK_DATA: NULL
*************************** 4. row ***************************
ENGINE: INNODB
ENGINE_LOCK_ID: 139727275781640:52:4:3:139724883013968
ENGINE_TRANSACTION_ID: 3790
THREAD_ID: 9441
EVENT_ID: 5007
OBJECT_SCHEMA: testdb
OBJECT_NAME: t
PARTITION_NAME: NULL
SUBPARTITION_NAME: NULL
INDEX_NAME: PRIMARY
OBJECT_INSTANCE_BEGIN: 139724883013968
LOCK_TYPE: RECORD
LOCK_MODE: X,REC_NOT_GAP
LOCK_STATUS: GRANTED
LOCK_DATA: 8
4 rows in set (0.00 sec)
说明:事务3795正在等待LOCK_TYPE: RECORD,LOCK_MODE:X,REC_NOT_GAP,等待的主键值为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 9320
sql_kill_blocking_connection: KILL 9320
1 row in set (0.01 sec)
说明:事务3795等待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: NULL
last_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

说明: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)。

说明:

MySQL的隔离级别是通过索引上的锁实现并发事务控制的。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,以禁止插入相同主键的数据;session2如果同时插入相同的主键数据被阻塞,容易理解(Oracle也同样阻塞)。出于同样的原因session2执行update时,由于无法获取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test1
mysql> 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_length
mysql> 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: 22593536
Max_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_length
mysql> 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: 16384
Max_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)

检查数据文件的大小
[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

说明:

MySQL在执行insert 语句进行数据插入,未提交时,数据也已写入表的聚集索引,辅助索引也已生成。MySQL可以使用未提交数据的索引,通过锁机制实现事务的并发控制。

3.Oracle中insert没有阻塞update

在Oracle中,创建同样的测试表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;

执行相同的insert和update语句。

session1session2
SQL> insert into t values(8,8);
1 row created.SQL> update t set b=0 where a=8;0 rows updated.

Enjoy GreatSQL :)


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