一条简单的SQL的加锁实现分析

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#数据库

本文详细探讨了MySQL/InnoDB中不同隔离级别下SQL语句的锁行为，特别是在Repeatable Read和Serializable隔离级别中，针对不同类型的索引（如主键、唯一索引和非唯一索引）以及无索引情况下，SQL执行时加锁的不同表现。

从之前笔记中摘录的，当时应该是根据何专家的博客整理的，发表时没有做审核，抱歉！

何专家博客地址：http://hedengcheng.com/?p=771

看一下下面的SQL语句加什么锁

SLQ1：select * from t1 where id = 10;

SQL2：delete from t1 where id = 10;

（1）id 是不是主键

（2）当前系统的隔离级别是什么

（3）id列如果不是主键，那么id列上有索引吗

（4）id列上如果有二级索引，那么这个索引是二级索引吗

（5）两个SQL的执行计划是什么？索引扫描还是全表扫描

实际的执行计划需要根据MySQL的输出为准

组合一：id列是主键，RC隔离级别
组合二：id列是二级唯一索引，RC隔离级别
组合三：id列是二级非唯一索引，RC隔离级别
组合四：id列没有索引，RC隔离级别
组合五：id列是主键，RR隔离级别
组合六：id列是二级唯一索引，RR隔离级别
组合七：id列是二级非唯一索引，RR隔离级别
组合八：id列上没有索引，RR隔离级别
Serializable隔离级别

在RR RC隔离级别下，SQL1：select 均不加锁，采用的是快照读；以下仅讨论SQL2：delete操作的加锁

Percona

组合一：id主键+RC

Percona

---TRANSACTION 1286310, ACTIVE 9 sec
2 lock struct(s), heap size 360, 1 row lock(s), undo log entries 1
MySQL thread id 341, OS thread handle 0x7f4d540d0700, query id 4510972 localhost root cleaning up
TABLE LOCK table `test`.`t1` trx id 1286310 lock mode IX
RECORD LOCKS space id 29 page no 3 n bits 80 index `PRIMARY` of table `test`.`t1` trx id 1286310 lock_mode X locks rec but not gap

MySQL

---TRANSACTION 5936, ACTIVE 171 sec
2 lock struct(s), heap size 360, 1 row lock(s), undo log entries 1
MySQL thread id 2, OS thread handle 0x7f5677201700, query id 364 localhost root
TABLE LOCK table `test`.`t1` trx id 5936 lock mode IX
RECORD LOCKS space id 6 page no 3 n bits 80 index `PRIMARY` of table `test`.`t1` trx id 5936 lock_mode X locks rec but not gap
Record lock, heap no 5 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 4; hex 8000000a; asc     ;;
 1: len 6; hex 000000001730; asc      0;;
 2: len 7; hex 26000001550110; asc &   U  ;;
 3: len 1; hex 61; asc a;;

组合二：id唯一索引+RC

在唯一索引上的更新需要两个X锁，一个对应唯一索引id=10 记录，一个对应于聚簇索引name='d'的记录

Percona

---TRANSACTION 1286327, ACTIVE 3 sec
3 lock struct(s), heap size 360, 2 row lock(s), undo log entries 1
MySQL thread id 344, OS thread handle 0x7f4d5404e700, query id 4510986 localhost root cleaning up
TABLE LOCK table `test`.`t2` trx id 1286327 lock mode IX
RECORD LOCKS space id 30 page no 4 n bits 80 index `id` of table `test`.`t2` trx id 1286327 lock_mode X locks rec but not gap
RECORD LOCKS space id 30 page no 3 n bits 80 index `PRIMARY` of table `test`.`t2` trx id 1286327 lock_mode X locks rec but not gap

MySQL

---TRANSACTION 5938, ACTIVE 3 sec
3 lock struct(s), heap size 360, 2 row lock(s), undo log entries 1
MySQL thread id 2, OS thread handle 0x7f5677201700, query id 374 localhost root
TABLE LOCK table `test`.`t2` trx id 5938 lock mode IX
RECORD LOCKS space id 7 page no 4 n bits 80 index `id` of table `test`.`t2` trx id 5938 lock_mode X locks rec but not gap
Record lock, heap no 7 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 8000000a; asc     ;;
 1: len 1; hex 64; asc d;;

RECORD LOCKS space id 7 page no 3 n bits 80 index `PRIMARY` of table `test`.`t2` trx id 5938 lock_mode X locks rec but not gap
Record lock, heap no 7 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 1; hex 64; asc d;;
 1: len 6; hex 000000001732; asc      2;;
 2: len 7; hex 27000001560110; asc '   V  ;;
 3: len 4; hex 8000000a; asc     ;;

组合三：id非唯一索引+RC

ID列为普通索引，那么对应的所有满足SQL查询条件的记录，都会被加锁；同时，这些记录在主键索引上的记录，也会被加锁

Percona

---TRANSACTION 1286339, ACTIVE 9 sec
3 lock struct(s), heap size 360, 4 row lock(s), undo log entries 2
MySQL thread id 347, OS thread handle 0x7f4b67fff700, query id 4511015 localhost root cleaning up
TABLE LOCK table `test`.`t3` trx id 1286339 lock mode IX
RECORD LOCKS space id 31 page no 4 n bits 80 index `idx_key` of table `test`.`t3` trx id 1286339 lock_mode X locks rec but not gap
RECORD LOCKS space id 31 page no 3 n bits 80 index `PRIMARY` of table `test`.`t3` trx id 1286339 lock_mode X locks rec but not gap

MySQL

---TRANSACTION 5940, ACTIVE 3 sec
3 lock struct(s), heap size 360, 4 row lock(s), undo log entries 2
MySQL thread id 2, OS thread handle 0x7f5677201700, query id 378 localhost root
TABLE LOCK table `test`.`t3` trx id 5940 lock mode IX
RECORD LOCKS space id 8 page no 4 n bits 80 index `idx_key` of table `test`.`t3` trx id 5940 lock_mode X locks rec but not gap
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 8000000a; asc     ;;
 1: len 1; hex 62; asc b;;

Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 8000000a; asc     ;;
 1: len 1; hex 64; asc d;;

RECORD LOCKS space id 8 page no 3 n bits 80 index `PRIMARY` of table `test`.`t3` trx id 5940 lock_mode X locks rec but not gap
Record lock, heap no 4 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 1; hex 62; asc b;;
 1: len 6; hex 000000001734; asc      4;;
 2: len 7; hex 28000001570110; asc (   W  ;;
 3: len 4; hex 8000000a; asc     ;;

Record lock, heap no 5 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 1; hex 64; asc d;;
 1: len 6; hex 000000001734; asc      4;;
 2: len 7; hex 28000001570132; asc (   W 2;;
 3: len 4; hex 8000000a; asc     ;;

组合四：id无索引+RC

Percona

---TRANSACTION 1286373, ACTIVE 5 sec
2 lock struct(s), heap size 360, 2 row lock(s), undo log entries 2
MySQL thread id 348, OS thread handle 0x7f4d54193700, query id 4511037 localhost root cleaning up
TABLE LOCK table `test`.`t4` trx id 1286373 lock mode IX
RECORD LOCKS space id 33 page no 3 n bits 80 index `PRIMARY` of table `test`.`t4` trx id 1286373 lock_mode X locks rec but not gap

MySQL

---TRANSACTION 5946, ACTIVE 2 sec
2 lock struct(s), heap size 360, 2 row lock(s), undo log entries 2
MySQL thread id 2, OS thread handle 0x7f5677201700, query id 382 localhost root
TABLE LOCK table `test`.`t4` trx id 5946 lock mode IX
RECORD LOCKS space id 9 page no 3 n bits 80 index `PRIMARY` of table `test`.`t4` trx id 5946 lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 1; hex 62; asc b;;
 1: len 6; hex 00000000173a; asc      :;;
 2: len 7; hex 2b0000015a0110; asc +   Z  ;;
 3: len 4; hex 8000000a; asc     ;;

Record lock, heap no 5 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 1; hex 64; asc d;;
 1: len 6; hex 00000000173a; asc      :;;
 2: len 7; hex 2b0000015a012c; asc +   Z ,;;
 3: len 4; hex 8000000a; asc     ;;

组合五：id主键+RR

参考组合一

组合六：id唯一索引+RR

参考组合二

组合七：id非唯一索引+RR

Percona

---TRANSACTION 1592633, ACTIVE 24 sec
4 lock struct(s), heap size 1184, 5 row lock(s), undo log entries 2
MySQL thread id 794, OS thread handle 0x7f4d5404e700, query id 7801799 localhost root cleaning up
Trx read view will not see trx with id >= 1592634, sees < 1592634
TABLE LOCK table `test`.`t3` trx id 1592633 lock mode IX
RECORD LOCKS space id 31 page no 4 n bits 80 index `idx_key` of table `test`.`t3` trx id 1592633 lock_mode X
RECORD LOCKS space id 31 page no 3 n bits 80 index `PRIMARY` of table `test`.`t3` trx id 1592633 lock_mode X locks rec but not gap
RECORD LOCKS space id 31 page no 4 n bits 80 index `idx_key` of table `test`.`t3` trx id 1592633 lock_mode X locks gap before rec

MySQL

---TRANSACTION 5985, ACTIVE 7 sec
4 lock struct(s), heap size 1184, 5 row lock(s), undo log entries 2
MySQL thread id 12, OS thread handle 0x7f56770fd700, query id 500 localhost root
TABLE LOCK table `test`.`t3` trx id 5985 lock mode IX
RECORD LOCKS space id 8 page no 4 n bits 80 index `idx_key` of table `test`.`t3` trx id 5985 lock_mode X
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 8000000a; asc     ;;
 1: len 1; hex 64; asc d;;

Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 8000000a; asc     ;;
 1: len 1; hex 62; asc b;;

RECORD LOCKS space id 8 page no 3 n bits 80 index `PRIMARY` of table `test`.`t3` trx id 5985 lock_mode X locks rec but not gap
Record lock, heap no 4 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 1; hex 64; asc d;;
 1: len 6; hex 000000001761; asc      a;;
 2: len 7; hex 3f0000016d0132; asc ?   m 2;;
 3: len 4; hex 8000000a; asc     ;;

Record lock, heap no 5 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 1; hex 62; asc b;;
 1: len 6; hex 000000001761; asc      a;;
 2: len 7; hex 3f0000016d0110; asc ?   m  ;;
 3: len 4; hex 8000000a; asc     ;;

RECORD LOCKS space id 8 page no 4 n bits 80 index `idx_key` of table `test`.`t3` trx id 5985 lock_mode X locks gap before rec
Record lock, heap no 8 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 4; hex 8000000b; asc     ;;
 1: len 1; hex 66; asc f;;

组合八：id无索引+RR

Percona

---TRANSACTION 1592639, ACTIVE 4 sec
2 lock struct(s), heap size 360, 7 row lock(s), undo log entries 2
MySQL thread id 794, OS thread handle 0x7f4d5404e700, query id 7801804 localhost root cleaning up
TABLE LOCK table `test`.`t4` trx id 1592639 lock mode IX
RECORD LOCKS space id 33 page no 3 n bits 80 index `PRIMARY` of table `test`.`t4` trx id 1592639 lock_mode X

MySQL

---TRANSACTION 6000, ACTIVE 3 sec
2 lock struct(s), heap size 360, 7 row lock(s), undo log entries 2
MySQL thread id 12, OS thread handle 0x7f56770fd700, query id 546 localhost root
TABLE LOCK table `test`.`t4` trx id 6000 lock mode IX
RECORD LOCKS space id 9 page no 3 n bits 80 index `PRIMARY` of table `test`.`t4` trx id 6000 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 1; hex 61; asc a;;
 1: len 6; hex 000000001722; asc      ";;
 2: len 7; hex 9e0000014e0110; asc     N  ;;
 3: len 4; hex 8000000f; asc     ;;

Record lock, heap no 3 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 1; hex 62; asc b;;
 1: len 6; hex 000000001770; asc      p;;
 2: len 7; hex 47000001730110; asc G   s  ;;
 3: len 4; hex 8000000a; asc     ;;

Record lock, heap no 4 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 1; hex 63; asc c;;
 1: len 6; hex 000000001722; asc      ";;
 2: len 7; hex 9e0000014e0122; asc     N ";;
 3: len 4; hex 80000006; asc     ;;

Record lock, heap no 5 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 1; hex 64; asc d;;
 1: len 6; hex 000000001770; asc      p;;
 2: len 7; hex 4700000173012c; asc G   s ,;;
 3: len 4; hex 8000000a; asc     ;;

Record lock, heap no 6 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 1; hex 66; asc f;;
 1: len 6; hex 000000001722; asc      ";;
 2: len 7; hex 9e0000014e0134; asc     N 4;;
 3: len 4; hex 8000000b; asc     ;;

Record lock, heap no 7 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 2; hex 7a7a; asc zz;;
 1: len 6; hex 000000001722; asc      ";;
 2: len 7; hex 9e0000014e013d; asc     N =;;
 3: len 4; hex 80000002; asc     ;;

组合九：Serializable

针对前面提到的简单的SQL，最后一个情况：Serializable隔离级别。对于SQL2：delete from t1 where id = 10; 来说，Serializable隔离级别与Repeatable Read隔离级别完全一致，因此不做介绍。

Serializable隔离级别，影响的是SQL1：select * from t1 where id = 10; 这条SQL，在RC，RR隔离级别下，都是快照读，不加锁。但是在Serializable隔离级别，SQL1会加读锁，也就是说快照读不复存在，MVCC并发控制降级为Lock-Based CC。

结论：在MySQL/InnoDB中，所谓的读不加锁，并不适用于所有的情况，而是隔离级别相关的。Serializable隔离级别，读不加锁就不再成立，所有的读操作，都是当前读。