摘要： MySQL兩條SQL語句鎖的分析
看一下下面的SQL語句加什么鎖
SLQ1：select * from t1 where id = 10;
SQL2：delete from t1 where id = 10;
（1）id 是不是主鍵
（2）當前系統的隔離級別是什么
（3）id列如果不是主鍵，那么id列上有索引嗎
（4）id列上如果有二級索引，那么這個索引是二級索引嗎
（5）兩個SQL的執行計劃是什么？索引掃描還是全表掃描
實際的執行計劃需要根據MySQL的輸出為準
組合一：id列是主鍵，RC隔離級別
組合二：id列是二級唯一索引，RC隔離級別
組合三：id列是二級非唯一索引，RC隔離級別
組合四：id列沒有索引，RC隔離級別
組合五：id列是主鍵，RR隔離級別
組合六：id列是二級唯一索引，RR隔離級別
組合七：id列是二級非唯一索引，RR隔離級別
組合八：id列上沒有索引，RR隔離級別
Serializable隔離級別
在RR RC隔離級別下，SQL1：select 均不加鎖，采用的是快照讀；以下僅討論SQL2：delete操作的加鎖
Percona
組合一：id主鍵+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;;
組合二：id唯一索引+RC
在唯一索引上的更新需要兩個X鎖，一個對應唯一索引id=10 記錄，一個對應于聚簇索引name='d'的記錄
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 ;;
組合三：id非唯一索引+RC
ID列為普通索引，那么對應的所有滿足SQL查詢條件的記錄，都會被加鎖；同時，這些記錄在主鍵索引上的記錄，也會被加鎖
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 ;;
組合四：id無索引+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 ;;
組合五：id主鍵+RR
參考 組合一
組合六：id唯一索引+RR
參考 組合二
組合七：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;;
組合八：id無索引+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 ;;
組合九：Serializable
針對前面提到的簡單的SQL，最后一個情況：Serializable隔離級別。對于SQL2：delete from t1 where id = 10; 來說，Serializable隔離級別與Repeatable Read隔離級別完全一致，因此不做介紹。
Serializable隔離級別，影響的是SQL1：select * from t1 where id = 10; 這條SQL，在RC，RR隔離級別下，都是快照讀，不加鎖。但是在Serializable隔離級別，SQL1會加讀鎖，也就是說快照讀不復存在，MVCC并發控制降級為Lock-Based CC。
結論：在MySQL/InnoDB中，所謂的讀不加鎖，并不適用于所有的情況，而是隔離級別相關的。Serializable隔離級別，讀不加鎖就不再成立，所有的讀操作，都是當前讀。
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