不同于Oracle：MySQL的insert會阻塞update

作者：王慶勛 2023-03-27 08:17:48

本文通過復(fù)現(xiàn)該問題，分析MySQL的鎖信息，確認是MySQL與Oracle在并發(fā)事務(wù)處理上的差異，在進行數(shù)據(jù)庫遷移改造的程序開發(fā)應(yīng)予以關(guān)注。

某銀行客戶在從Oracle遷移到MySQL的開發(fā)中，MySQL在READ-COMMITTED隔離級別下，出現(xiàn)了insert阻塞update的情況，但同樣的情況下，Oracle的insert則不會阻塞update。本文通過復(fù)現(xiàn)該問題，分析MySQL的鎖信息，確認是MySQL與Oracle在并發(fā)事務(wù)處理上的差異，在進行數(shù)據(jù)庫遷移改造的程序開發(fā)應(yīng)予以關(guān)注。

1.問題復(fù)現(xiàn)

1.1.環(huán)境準備

MySQL版本的8.0.26，隔離級別是READ-COMMITTED ，測試表t的字段a為主鍵。

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的操作步驟

insert語句未提交時，update同樣主鍵的數(shù)據(jù)會被阻塞。

session1	session2
插入一條數(shù)據(jù)(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)
	更改數(shù)據(jù)，條件是a=8,將會被阻塞 mysql> update t set b=0 where a=8; <<掛起，等待innodb_lock_wait_timeout超時

2.分析原因

2.1.檢查事務(wù)鎖信息

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)

說明：通過InnoDB的事務(wù)表innodb_trx查詢到thread_id=9346的事務(wù)3795正在等待鎖（trx_state: LOCK WAIT）,
thread_id=9320的事務(wù)3790正在執(zhí)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)
說明：事務(wù)3795正在等待LOCK_TYPE: RECORD，LOCK_MODE:X,REC_NOT_GAP，等待的主鍵值為8；
事務(wù)3790已獲取主鍵值為8的LOCK_TYPE: RECORD，LOCK_MODE:X,REC_NOT_GAP鎖。

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)
說明：事務(wù)3795等待testdb.t上的rec_not_gap獨占鎖，事務(wù)3790持有該獨占鎖。
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
說明：被阻塞事務(wù)執(zhí)行的sql語句update t set b=0 where a=8，
阻塞事務(wù)執(zhí)行的sql語句是insert into t values(8,8)。

說明：

MySQL的隔離級別是通過索引上的鎖實現(xiàn)并發(fā)事務(wù)控制的。在READ-COMMITTED隔離級別下，session1在執(zhí)行insert語句時，在主鍵索引上獲取了a=8的行記錄獨占鎖，以禁止插入相同主鍵的數(shù)據(jù)；session2如果同時插入相同的主鍵數(shù)據(jù)被阻塞，容易理解（Oracle也同樣阻塞）。出于同樣的原因session2執(zhí)行update時，由于無法獲取a=8的行記錄獨占鎖，同樣也會被阻塞。

2.2.驗證MySQL事務(wù)未提交時已寫入數(shù)據(jù)文件

驗證事務(wù)未提交時，insert語句已將數(shù)據(jù)寫入數(shù)據(jù)文件，索引數(shù)據(jù)也已生成。

測試表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)

開啟一個事務(wù)，插入10萬條數(shù)據(jù)。
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

檢查表的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)

回滾insert操作
mysql> rollback;
Query OK, 0 rows affected (1.35 sec)

更新統(tǒng)計信息
mysql> analyze table test1;

再次檢查表的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)

檢查數(shù)據(jù)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

說明：

MySQL在執(zhí)行insert 語句進行數(shù)據(jù)插入，未提交時，數(shù)據(jù)也已寫入表的聚集索引，輔助索引也已生成。MySQL可以使用未提交數(shù)據(jù)的索引，通過鎖機制實現(xiàn)事務(wù)的并發(fā)控制。

3.Oracle中insert沒有阻塞update

在Oracle中，創(chuàng)建同樣的測試表t，執(zhí)行同樣的insert和update，但insert不會阻塞update。

CREATE TABLE t (
  a int NOT NULL PRIMARY KEY ,
  b int DEFAULT NULL
);
insert into t values(7,7);
commit;

執(zhí)行相同的insert和update語句。

session1	session2
SQL> insert into t values(8,8);
1 row created.	SQL> update t set b=0 where a=8;0 rows updated.

責(zé)任編輯：武曉燕來源： GreatSQL社區(qū)

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