For Update 加鎖分析

GeaoZhang發表於2022-01-24

MySQL InnoDB 鎖 - For Update 加鎖分析：

1. InnoDB鎖簡單介紹

2. 當前讀加鎖分析：REPEATABLE-READ 可重複讀、READ-COMMITTED 讀已提交

3. 鎖模式說明及8.0的data_locks表

一、InnoDB鎖

1、全域性鎖

全域性讀鎖，flush tables with read lock，整庫處於只讀狀態。全域性鎖的一個典型場景：全庫邏輯備份，--single-transaction實現一致性讀。

2、表級鎖

表鎖，lock tables…read/write，主動在表上加讀鎖或寫鎖；

後設資料鎖（meta data lock，MDL），訪問表時自動加上，防止DDL和DML併發的衝突，保證讀寫正確性；

自增鎖，表中auto_increment欄位的自增值生成控制，記憶體自增計數器，8.0之後通過redo進行持久化；

意向鎖，只會和表級鎖發生衝突，不會阻止除表鎖請求之外的鎖，表明有事務即將、或正鎖定某N行；

意向共享鎖（IS）：SELECT ... LOCK IN SHARE MODE，在對應記錄行上加鎖之前，在表上加意向共享鎖；
意向排它鎖（IX）：SELECT .. FOR UPDATE，悲觀鎖，對錶所有掃描過的行都會被加上意向排它鎖，若掃描行其中有行鎖，則會被阻塞；對SELECT索引加排它鎖，阻塞其他事務修改或SELECT ... FOR SHARE（在8.0新增的方式）；

3、行鎖

行鎖，事務鎖，只有發生行鎖衝突，才會出現事務鎖的排隊等待。

兩階段鎖協議：行鎖在需要時加上，事務結束時釋放。

行鎖的3種演算法：record lock、gap lock、next-key lock

記錄鎖 record lock：新增在索引上，表中沒有索引時會預設新增在預設建立的聚集索引上；
間隙鎖 gap lock：鎖定一個範圍，可重複讀 隔離級別下，行鎖會變成gap鎖（範圍鎖），降低併發性，當前讀(dml、select for update)，若where條件列上有索引，加gap lock在索引上，實現可重複讀；
Next-Key Lock：Gap Lock+Record Lock，鎖定一個範圍，並且鎖定記錄本身；

　　　　① 唯一索引或主鍵，Next-Key Lock 降為 Record Lock，即僅鎖住索引本身，而不是範圍。

　　　　② 輔助索引（二級索引），預設使用Next-Key Locking加鎖，鎖定範圍是前一個索引到後一個索引之間範圍，左開右閉。

### session 1
root@test 15:51 > begin;

root@test 15:51 > show create table student;
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                                                   |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| student | CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `birthday` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `ix_name` (`name`),
  KEY `ix_birthday` (`birthday`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

root@test 15:52 > select * from student;
+----+-------+---------------------+
| id | name  | birthday            |
+----+-------+---------------------+
|  1 | abcd  | 1995-06-27 00:00:00 |
|  2 | abef  | 1995-01-24 00:00:00 |
|  3 | abg   | 1995-07-26 00:00:00 |
|  4 | cdmn  | 1995-06-13 00:00:00 |
+----+-------+---------------------+

root@test 15:52 > select * from student where birthday > '1995-06-27 00:00:00' and birthday < '1995-07-26 00:00:00' for update;
Empty set (0.02 sec)

### session 2
root@test 15:51 > begin;

# 左開
root@test 15:54 > update student set name = 'abcd' where birthday = "1995-06-27 00:00:00";

# 右閉(阻塞更新)
root@test 15:55 > update student set name = 'abg' where birthday = '1995-07-26 00:00:00';
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

二、當前讀加鎖分析

分析不同隔離級別，當前讀（dml、select … for update）的加鎖情況

1、REPEATABLE-READ 可重複讀

root@test 15:06 > show global variables like "%iso%";
+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
| tx_isolation          | REPEATABLE-READ |
+-----------------------+-----------------+

1.1、表無顯式主鍵和索引

root@test 15:30 > show create table t_student;
+-----------+----------------------------------------------------------------------------------------------------------------------------+
| Table     | Create Table                                                                                                               |
+-----------+----------------------------------------------------------------------------------------------------------------------------+
| t_student | CREATE TABLE `t_student` (
  `id` int(11) NOT NULL,
  `name` varchar(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-----------+----------------------------------------------------------------------------------------------------------------------------+

root@test 15:30 > begin;

root@test 15:31 > select * from t_student for update;
+----+-------+
| id | name  |
+----+-------+
|  1 | jack  |
|  2 | kuzma |
|  3 | linda |
+----+-------+

如上，可重複讀，表上沒有主鍵（會自動生成隱式主鍵聚集組織表），也沒有索引，全表SELECT的當前讀的加鎖情況：

　　① 對錶新增 IX 鎖

　　② 在"supremum"上新增 Next-Key Lock（supremum表示高於表中任何一個索引的值），即最大索引值之後的間隙鎖住

　　③ 在三條記錄上分別新增 Next-Key Lock

root@test 16:08 > begin;

# where上帶條件 id = 3
root@test 16:08 > select * from t_student where id = 3 for update;
+----+-------+
| id | name  |
+----+-------+
|  3 | linda |
+----+-------+

root@test 16:09 > show engine innodb status\G
# 鎖資訊如上，表上IX，supremum 和 三條記錄上新增 Next-Key Lock

# 另開一個session
root@test 16:09 > begin;

root@test 16:13 > insert into t_student values(2,"tom");
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
root@test 16:15 > insert into t_student values(4,"tom");
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

如上，insert為什麼會被阻塞呢？delete呢？update呢？

每次插入記錄時，所生成的聚集索引(DB_ROW_ID)是自增的，每次都會在表的最後插入，因此就有可能插入id=10這條記錄，所以需要新增"supremum pseudo-record"防止資料插入。

---TRANSACTION 582122, ACTIVE 3874 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 1136, 6 row lock(s)
MySQL thread id 12529, OS thread handle 123145486712832, query id 94463 localhost root update
insert into t_student values(5,"tom")
Trx read view will not see trx with id >= 582123, sees < 582121
------- TRX HAS BEEN WAITING 3 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 168 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t_student` trx id 582122 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

------------------
TABLE LOCK table `test`.`t_student` trx id 582122 lock mode IX
RECORD LOCKS space id 168 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t_student` trx id 582122 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

即使是不滿足where條件的記錄上，也會新增Next-Key Lock，目的是為了防止幻讀。因此的，其他會話事務執行delete或者update都會造成幻讀，也就被阻塞的。

---TRANSACTION 582122, ACTIVE 3788 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 1136, 5 row lock(s)
MySQL thread id 12529, OS thread handle 123145486712832, query id 94461 localhost root updating
update t_student set name = "linda" where id = 3
Trx read view will not see trx with id >= 582123, sees < 582121
------- TRX HAS BEEN WAITING 14 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 168 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t_student` trx id 582122 lock_mode X waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 6; hex 00000000090a; asc       ;;
 1: len 6; hex 00000008e1c8; asc       ;;
 2: len 7; hex f4000001e40110; asc        ;;
 3: len 4; hex 80000001; asc     ;;
 4: len 4; hex 6a61636b; asc jack;;

------------------
TABLE LOCK table `test`.`t_student` trx id 582122 lock mode IX
RECORD LOCKS space id 168 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t_student` trx id 582122 lock_mode X waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 6; hex 00000000090a; asc       ;;
 1: len 6; hex 00000008e1c8; asc       ;;
 2: len 7; hex f4000001e40110; asc        ;;
 3: len 4; hex 80000001; asc     ;;
 4: len 4; hex 6a61636b; asc jack;;

1.2、表有顯式主鍵無索引

不帶where條件，表上IX，主鍵最大索引值之後的間隙鎖住，表所有記錄對應主鍵加上記錄鎖
where條件是主鍵，表上IX，主鍵新增記錄鎖（X，REC_NOT_GAP），並且只鎖住條件記錄，因為主鍵保證唯一，不需要間隙鎖

---TRANSACTION 582139, ACTIVE 12 sec
2 lock struct(s), heap size 1136, 1 row lock(s)
MySQL thread id 12528, OS thread handle 123145486434304, query id 94472 localhost root
TABLE LOCK table `test`.`t_student` trx id 582139 lock mode IX
RECORD LOCKS space id 169 page no 3 n bits 72 index PRIMARY of table `test`.`t_student` trx id 582139 lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 4; hex 80000002; asc     ;;
 1: len 6; hex 00000008e1c8; asc       ;;
 2: len 7; hex f4000001e4011f; asc        ;;
 3: len 5; hex 6b757a6d61; asc kuzma;;

1.3、表無顯式主鍵有索引

不帶where條件，表上IX，id最大索引值之後的間隙鎖住，表所有記錄對應索引加上記錄鎖
where條件是普通索引欄位

root@test 22:01 > show create table t_student;
+-----------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| Table     | Create Table                                                                                                                                     |
+-----------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| t_student | CREATE TABLE `t_student` (
  `id` int(11) NOT NULL,
  `name` varchar(10) DEFAULT NULL,
  KEY `ix_id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-----------+--------------------------------------------------------------------------------------------------------------------------------------------------+

root@test 22:10 > begin;

root@test 22:11 > select * from t_student where id = 2 for update;
+----+-------+
| id | name  |
+----+-------+
|  2 | kuzma |
+----+-------+

root@test 22:11 > show engine innodb status\G
---TRANSACTION 582176, ACTIVE 14 sec
4 lock struct(s), heap size 1136, 3 row lock(s)
MySQL thread id 12534, OS thread handle 123145487269888, query id 94485 localhost root
TABLE LOCK table `test`.`t_student` trx id 582176 lock mode IX
RECORD LOCKS space id 170 page no 4 n bits 72 index ix_id of table `test`.`t_student` trx id 582176 lock_mode X
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 4; hex 80000002; asc     ;;
 1: len 6; hex 000000000912; asc       ;;

RECORD LOCKS space id 170 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t_student` trx id 582176 lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 6; hex 000000000912; asc       ;;
 1: len 6; hex 00000008e200; asc       ;;
 2: len 7; hex be00000146011f; asc     F  ;;
 3: len 4; hex 80000002; asc     ;;
 4: len 5; hex 6b757a6d61; asc kuzma;;

RECORD LOCKS space id 170 page no 4 n bits 72 index ix_id of table `test`.`t_student` trx id 582176 lock_mode X locks gap before rec
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 4; hex 80000003; asc     ;;
 1: len 6; hex 000000000913; asc       ;;

如上，過濾一個普通索引時的加鎖情況是：

　　① 對錶新增 IX鎖

　　② 對id=2對應的索引新增 Next-Key Lock鎖，區間是(-∞, 2]

　　③ 對索引對應的聚集索引新增 X記錄鎖

　　④ 為防止幻讀（因為是普通索引，可重複插入id=2的記錄），對索引記錄區間(2,3)新增間隙鎖（lock_mode X locks gap before rec）

也就是說，（測試用例有些許不恰當）如果說放大到10~20~30測試，id直到29之間的記錄插入都會被阻塞，而插入id=30因為不在間隙鎖範圍，不會被阻塞。

1.4、表有顯式主鍵和索引

無where條件，如1.2.1描述
有where條件，其實，無論是主鍵，還是唯一鍵索引，只要能準確定位的，都是表上IX，主鍵或唯一鍵新增記錄鎖（X，REC_NOT_GAP），並且只鎖住條件記錄

2、READ-COMMITTED 讀已提交

root@test 23:57 > show global variables like "tx_isolation";
+---------------+----------------+
| Variable_name | Value          |
+---------------+----------------+
| tx_isolation  | READ-COMMITTED |
+---------------+----------------+

2.1、表無顯式主鍵和索引

root@test 23:57 > show create table t_people;
+----------+-------------------------------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                                                  |
+----------+-------------------------------------------------------------------------------------------------------------------------------+
| t_people | CREATE TABLE `t_people` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+----------+-------------------------------------------------------------------------------------------------------------------------------+

root@test 23:58 > select * from t_people;
+------+-------+
| id   | name  |
+------+-------+
|    1 | jack  |
|    2 | kuzma |
|    3 | linda |
+------+-------+

如下，表t_people上加 IX意向排它鎖，表中所有行的隱藏主鍵上（行格式裡第三個就是隱藏主鍵，在表中沒有顯示主鍵的時候自動生成用於組織表資料）加 X記錄鎖，只鎖定記錄本身，非範圍間隙鎖。

如果帶上where條件呢？

root@test 00:44 > begin;

root@test 00:45 > select * from t_people where id = 2 for update;
+------+-------+
| id   | name  |
+------+-------+
|    2 | kuzma |
+------+-------+

root@test 00:45 > show engine innodb status;
......
---TRANSACTION 582053, ACTIVE 4 sec
2 lock struct(s), heap size 1136, 1 row lock(s)
MySQL thread id 12521, OS thread handle 123145487269888, query id 94339 localhost root
TABLE LOCK table `test`.`t_people` trx id 582053 lock mode IX
RECORD LOCKS space id 166 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t_people` trx id 582053 lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 6; hex 000000000908; asc       ;;
 1: len 6; hex 00000008e19e; asc       ;;
 2: len 7; hex d2000001580110; asc     X  ;;
 3: len 4; hex 80000002; asc     ;;
 4: len 5; hex 6b757a6d61; asc kuzma;;
 ......
 # 只是在 id = 2 的記錄上加了X記錄鎖

2.2、表有顯式主鍵無索引

不帶where條件，如上的，表中所有行上加 X記錄鎖，不鎖定範圍
where條件是主鍵欄位時，對錶加 IX鎖，對主鍵新增記錄鎖(X, REC_NOTGAP)，where了主鍵，主鍵已經保證唯一，非範圍鎖，鎖加在具體的記錄上

root@test 00:49 > show engine innodb  status;
---TRANSACTION 582071, ACTIVE 7 sec
2 lock struct(s), heap size 1136, 1 row lock(s)
MySQL thread id 12521, OS thread handle 123145487269888, query id 94353 localhost root
TABLE LOCK table `test`.`t_people` trx id 582071 lock mode IX
RECORD LOCKS space id 167 page no 3 n bits 72 index PRIMARY of table `test`.`t_people` trx id 582071 lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 4; hex 80000002; asc     ;;
 1: len 6; hex 00000008e19e; asc       ;;
 2: len 7; hex d2000001580110; asc     X  ;;
 3: len 5; hex 6b757a6d61; asc kuzma;;

where條件包含主鍵欄位和非主鍵欄位，和上面是一樣的，因為主鍵已經定位了加鎖。

2.3、表無顯示主鍵有索引

不帶where條件，同樣的，表中所有行上加 X記錄鎖，不鎖定範圍
where條件是普通索引欄位，唯一索引亦如是

root@test 20:24 > show create table t_student;
+-----------+------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table     | Create Table                                                                                                                                         |
+-----------+------------------------------------------------------------------------------------------------------------------------------------------------------+
| t_student | CREATE TABLE `t_student` (
  `id` int(11) NOT NULL,
  `name` varchar(10) DEFAULT NULL,
  KEY `ix_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-----------+------------------------------------------------------------------------------------------------------------------------------------------------------+

root@test 20:25 > begin;

root@test 20:27 > select * from t_student where name = 'kuzma' for update;
+----+-------+
| id | name  |
+----+-------+
|  2 | kuzma |
+----+-------+

root@test 20:27 > show engine innodb status;
---TRANSACTION 582093, ACTIVE 57 sec
3 lock struct(s), heap size 1136, 2 row lock(s)
MySQL thread id 12525, OS thread handle 123145486712832, query id 94388 localhost root
TABLE LOCK table `test`.`t_student` trx id 582093 lock mode IX
RECORD LOCKS space id 168 page no 4 n bits 72 index ix_name of table `test`.`t_student` trx id 582093 lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 5; hex 6b757a6d61; asc kuzma;;
 1: len 6; hex 00000000090b; asc       ;;

RECORD LOCKS space id 168 page no 3 n bits 72 index GEN_CLUST_INDEX of table `test`.`t_student` trx id 582093 lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 6; hex 00000000090b; asc       ;;
 1: len 6; hex 00000008e1c8; asc       ;;
 2: len 7; hex f4000001e4011f; asc        ;;
 3: len 4; hex 80000002; asc     ;;
 4: len 5; hex 6b757a6d61; asc kuzma;;

如上，當where條件上是普通索引欄位時，加鎖情況如下：

　　① 表上加 IX，意向排它鎖

　　② name='kuzma' 對應的索引上新增 X 記錄鎖

　　③ GEN_CLUST_INDEX，對應的（自動生成的）聚集索引上新增 X 記錄鎖

2.4、表有顯式主鍵有索引

不帶where條件，所有行記錄新增記錄鎖，並沒有間隙範圍鎖
where條件是普通索引或主鍵索引，如前面的，先是表上加 IX 意向排它鎖，然後在對應的普通索引上新增 X 記錄鎖（如果是主鍵則無需），最後在對應的聚集索引（主鍵）上新增 X 記錄鎖

三、鎖模式說明及8.0的data_locks表

3.1、鎖模式LOCK_MODE

show engine innodb status中見到的 lock_mode，如下整理說明：

鎖模式	說明
IX	意向排它鎖
X	Next-Key Lock鎖定記錄和記錄之前的間隙(X)
S	Next-Key Lock鎖定記錄和記錄之前的間隙(S)
X, REC_NOT_GAP	只鎖定記錄本身(X)
S, REC_NOT_GAP	只鎖定記錄本身(S)
X, GAP	間隙鎖，不鎖定記錄本身(X)
S, GAP	間隙鎖，不鎖定記錄本身(S)
X, GAP, INSERT_INTENTION	插入意向鎖，間隙範圍，排它

3.2、8.0的data_locks表

在8.0之前，檢視鎖資訊可以通過設定innodb_status_output_locks=ON，然後在 show engine innodb status的輸出裡，看到詳細的鎖資訊列印，如前面展示。

8.0之後，performance_schema.data_locks，新增的，記錄表加鎖情況，可將之前的innodb status鎖資訊列印更好的以表格的形式記錄展示。

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