RR與RC模式下的間隙鎖導(dǎo)致的區(qū)別

隔離級(jí)別

MySQL默認(rèn)隔離級(jí)別為 REPEATABLE READ
常用的隔離級(jí)別為如下倆個(gè)：

  • REPEATABLE READ 可重復(fù)讀
  • READ COMMITTED 已提交讀

間隙鎖（REPEATABLE READ）

官方文檔：https://dev.mysql.com/doc/refman/5.7/en/innodb-locking.html#innodb-gap-locks

  • 間隙鎖定是對(duì)索引記錄之間的間隙的鎖定疹娶，或者是對(duì)第一個(gè)或最后一個(gè)索引記錄之前的間隙的鎖定净响。
  • 對(duì)于唯一索引來說，沒有間隙鎖（不包括搜索條件含多列唯一索引情況）。
  • 間隙鎖在 REPEATABLE READ 以上模式下才存在

實(shí)例展示

測(cè)試數(shù)據(jù)

環(huán)境：MySQL 8.0.18

【session1】
create database locktest;
drop table yqlock1;
create table yqlock1 (id int auto_increment primary key , a int ,b varchar(30),key idx_a(a));
insert into yqlock1(a,b) values (3,5),(5,12),(9,8),(7,1),(8,5),(15,20);

root@yq [locktest]> select * from yqlock1 order by a;
+----+------+------+
| id | a    | b    |
+----+------+------+
|  1 |    3 | 5    |
|  2 |    5 | 12   |
|  4 |    7 | 1    |
|  5 |    8 | 5    |
|  3 |    9 | 8    |
|  6 |   15 | 20   |
+----+------+------+
6 rows in set (0.00 sec)

RR隔離級(jí)別

條件字段有索引

# 查看隔離級(jí)別
root@yq [locktest]> show variables like '%iso%';
+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.01 sec)

begin;
update yqlock1 set b = '隔壁老王' where a = '5'; # a列存在索引

# 8.0 可以查看data_locks表查看鎖定的數(shù)據(jù)行
# 可以看到有一個(gè)間隙鎖 7, 4
root@yq [locktest]> select ENGINE,ENGINE_TRANSACTION_ID,THREAD_ID,EVENT_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| ENGINE | ENGINE_TRANSACTION_ID | THREAD_ID | EVENT_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE     | LOCK_STATUS | LOCK_DATA |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| INNODB |               2287449 |    155509 |       15 | locktest      | yqlock1     | NULL       | TABLE     | IX            | GRANTED     | NULL      |
| INNODB |               2287449 |    155509 |       15 | locktest      | yqlock1     | idx_a      | RECORD    | X             | GRANTED     | 5, 2      |
| INNODB |               2287449 |    155509 |       15 | locktest      | yqlock1     | PRIMARY    | RECORD    | X,REC_NOT_GAP | GRANTED     | 2         |
| INNODB |               2287449 |    155509 |       15 | locktest      | yqlock1     | idx_a      | RECORD    | X,GAP         | GRANTED     | 7, 4      |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
4 rows in set (0.00 sec)

# 【session2】新開一個(gè)窗口角钩，執(zhí)行insert
root@yq [locktest]> begin;
Query OK, 0 rows affected (0.00 sec)

root@yq [locktest]> insert into yqlock1 select 7,'5','aaa';  # 處于等待狀態(tài)
root@yq [locktest]> insert into yqlock1 select 7,'6','aaa';  # 處于等待狀態(tài)
root@yq [locktest]> insert into yqlock1 select 7,'7','aaa';  # 成功插入
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

在測(cè)試下更新的數(shù)據(jù)是不存在的記錄

【session1】
root@yq [locktest]> begin;
Query OK, 0 rows affected (0.00 sec)

root@yq [locktest]> update yqlock1 set b = '隔壁老王' where a = '12';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0  Changed: 0  Warnings: 0

root@yq [locktest]> select ENGINE,ENGINE_TRANSACTION_ID,THREAD_ID,EVENT_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+-----------+-------------+-----------+
| ENGINE | ENGINE_TRANSACTION_ID | THREAD_ID | EVENT_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+-----------+-------------+-----------+
| INNODB |               2287472 |    155663 |       25 | locktest      | yqlock1     | NULL       | TABLE     | IX        | GRANTED     | NULL      |
| INNODB |               2287472 |    155663 |       25 | locktest      | yqlock1     | idx_a      | RECORD    | X,GAP     | GRANTED     | 15, 6     |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+-----------+-------------+-----------+
2 rows in set (0.00 sec)

【session2】
root@yq [locktest]> begin;
Query OK, 0 rows affected (0.00 sec)
root@yq [locktest]> insert into yqlock1 select 7,'9','aaa';   ## 等待
root@yq [locktest]> insert into yqlock1 select 8,'10','aaa';  ## 等待
root@yq [locktest]> insert into yqlock1 select 9,'12','aaa';  ## 等待
root@yq [locktest]> insert into yqlock1 select 10,'15','aaa';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

由上示例可得知：

  • RR模式下存在間隙鎖（GAP），會(huì)鎖住兩索引間的間隙阀参。
  • 條件有索引情況下鸣皂，實(shí)際鎖住的數(shù)據(jù)是左閉右開(5,7],其實(shí)這是 next-key lock(行鎖+GAP)

條件字段無索引

【session1】
root@yq [locktest]> select * from yqlock1 order by b;
+----+------+------+
| id | a    | b    |
+----+------+------+
|  4 |    7 | 1    |
|  2 |    5 | 12   |
|  6 |   15 | 20   |
|  1 |    3 | 5    |
|  5 |    8 | 5    |
|  3 |    9 | 8    |
+----+------+------+
6 rows in set (0.00 sec)

begin;
root@yq [locktest]> update yqlock1 set a = 123 where b = '5';
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2  Changed: 2  Warnings: 0

root@yq [locktest]> select ENGINE,ENGINE_TRANSACTION_ID,THREAD_ID,EVENT_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+-----------+-------------+------------------------+
| ENGINE | ENGINE_TRANSACTION_ID | THREAD_ID | EVENT_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA              |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+-----------+-------------+------------------------+
| INNODB |               2287516 |    155889 |       18 | locktest      | yqlock1     | NULL       | TABLE     | IX        | GRANTED     | NULL                   |
| INNODB |               2287516 |    155889 |       18 | locktest      | yqlock1     | PRIMARY    | RECORD    | X         | GRANTED     | supremum pseudo-record |
| INNODB |               2287516 |    155889 |       18 | locktest      | yqlock1     | PRIMARY    | RECORD    | X         | GRANTED     | 1                      |
| INNODB |               2287516 |    155889 |       18 | locktest      | yqlock1     | PRIMARY    | RECORD    | X         | GRANTED     | 3                      |
| INNODB |               2287516 |    155889 |       18 | locktest      | yqlock1     | PRIMARY    | RECORD    | X         | GRANTED     | 4                      |
| INNODB |               2287516 |    155889 |       18 | locktest      | yqlock1     | PRIMARY    | RECORD    | X         | GRANTED     | 5                      |
| INNODB |               2287516 |    155889 |       18 | locktest      | yqlock1     | PRIMARY    | RECORD    | X         | GRANTED     | 6                      |
| INNODB |               2287516 |    155889 |       18 | locktest      | yqlock1     | PRIMARY    | RECORD    | X         | GRANTED     | 2                      |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+-----------+-------------+------------------------+
  • 條件無索引情況下摔踱，鎖全記錄虐先，并存在 supremum pseudo-record，也就是后續(xù)的也不能寫入（鎖全表）

RC隔離級(jí)別

條件字段有索引

【session1】
root@yq [locktest]> show variables like '%iso%';
+-----------------------+-----------------+
| Variable_name         | Value           |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

root@yq [locktest]> set session transaction_isolation = 'READ-COMMITTED';
Query OK, 0 rows affected (0.00 sec)

root@yq [locktest]> show variables like '%iso%';
+-----------------------+----------------+
| Variable_name         | Value          |
+-----------------------+----------------+
| transaction_isolation | READ-COMMITTED |
+-----------------------+----------------+
1 row in set (0.00 sec)

root@yq [locktest]> begin;
Query OK, 0 rows affected (0.00 sec)

root@yq [locktest]> update yqlock1 set b = '隔壁老王' where a = '5';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

root@yq [locktest]> select ENGINE,ENGINE_TRANSACTION_ID,THREAD_ID,EVENT_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| ENGINE | ENGINE_TRANSACTION_ID | THREAD_ID | EVENT_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE     | LOCK_STATUS | LOCK_DATA |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| INNODB |               2287524 |    156236 |        8 | locktest      | yqlock1     | NULL       | TABLE     | IX            | GRANTED     | NULL      |
| INNODB |               2287524 |    156236 |        8 | locktest      | yqlock1     | idx_a      | RECORD    | X,REC_NOT_GAP | GRANTED     | 5, 2      |
| INNODB |               2287524 |    156236 |        8 | locktest      | yqlock1     | PRIMARY    | RECORD    | X,REC_NOT_GAP | GRANTED     | 2         |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
3 rows in set (0.00 sec)

【session2】
root@yq [locktest]> begin;
Query OK, 0 rows affected (0.00 sec)

root@yq [locktest]> insert into yqlock1 select 7,'9','aaa';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

root@yq [locktest]> insert into yqlock1 select 8,'10','aaa';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

root@yq [locktest]> insert into yqlock1 select 9,'12','aaa';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

root@yq [locktest]> insert into yqlock1 select 10,'15','aaa';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0
  • 僅鎖住了相應(yīng)的條件行

更新的數(shù)據(jù)是不存在的記錄

【session1】
root@yq [locktest]> begin;
Query OK, 0 rows affected (0.00 sec)

root@yq [locktest]> update yqlock1 set b = '隔壁老王' where a = '12';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0  Changed: 0  Warnings: 0

root@yq [locktest]> select ENGINE,ENGINE_TRANSACTION_ID,THREAD_ID,EVENT_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+-----------+-------------+-----------+
| ENGINE | ENGINE_TRANSACTION_ID | THREAD_ID | EVENT_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+-----------+-------------+-----------+
| INNODB |               2287531 |    156236 |       13 | locktest      | yqlock1     | NULL       | TABLE     | IX        | GRANTED     | NULL      |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+-----------+-------------+-----------+
1 row in set (0.00 sec)

【session2】
root@yq [locktest]> begin;
Query OK, 0 rows affected (0.00 sec)

root@yq [locktest]> insert into yqlock1 select 10,'12','aaa';
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0
  • 僅有一個(gè)IX鎖派敷，不影響數(shù)據(jù)寫入蛹批，即使寫入的值是上述更新的值

條件字段無索引

root@yq [locktest]> begin;
Query OK, 0 rows affected (0.00 sec)

root@yq [locktest]> update yqlock1 set a = 123 where b = '5';
Query OK, 2 rows affected (0.02 sec)
Rows matched: 2  Changed: 2  Warnings: 0

root@yq [locktest]> select ENGINE,ENGINE_TRANSACTION_ID,THREAD_ID,EVENT_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| ENGINE | ENGINE_TRANSACTION_ID | THREAD_ID | EVENT_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE     | LOCK_STATUS | LOCK_DATA |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
| INNODB |               2287537 |    156236 |       18 | locktest      | yqlock1     | NULL       | TABLE     | IX            | GRANTED     | NULL      |
| INNODB |               2287537 |    156236 |       18 | locktest      | yqlock1     | PRIMARY    | RECORD    | X,REC_NOT_GAP | GRANTED     | 1         |
| INNODB |               2287537 |    156236 |       18 | locktest      | yqlock1     | PRIMARY    | RECORD    | X,REC_NOT_GAP | GRANTED     | 5         |
+--------+-----------------------+-----------+----------+---------------+-------------+------------+-----------+---------------+-------------+-----------+
3 rows in set (0.00 sec)
  • 可以看出僅對(duì)條件符合的行加鎖
  • 其實(shí)在RC模式下撰洗，剛開始是鎖了全記錄的，MySQL評(píng)估WHERE條件后腐芍，將釋放不匹配行的記錄鎖差导。
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