+------+------+------+------+
orignal | 10 | 20 | 30 | 40 |
unique +------+------+------+------+
delete 20 +------+------+------+------+
| 10 | 20* | 30 | 40 | (20 : delete-mark)
and commit +------+^----^+------+------+
| |
non happen | +--insert new 20
|
Purge
# 自增主键图中没有给出

[root@MyServer ~]> mysqld-debug --version
mysqld-debug Ver 5.7.11-debug for linux-glibc2.5 on x86_64 (MySQL Community Server - Debug (GPL))
[root@MyServer ~]> mysqld-debug --datadir=/data/mysql_data/5.7.11/ &
[1] 1493
[root@MyServer ~]> netstat -tunlp | grep 3306
tcp 0 0 0.0.0.0:3306 0.0.0.0:* LISTEN 1493/mysqld-debug
--
-- 终端会话1
mysql> create table test_purge(a int auto_increment primary key, b int , unique key(b));
Query OK, 0 rows affected (0.20 sec)
mysql> insert into test_purge(b) values (10),(20),(30),(40);
Query OK, 4 rows affected (0.05 sec)
mysql> commit; -- autocommit=0 in my.cnf
Query OK, 0 rows affected (0.03 sec)
mysql> set global innodb_purge_stop_now=1;
-- show这个变量，结果还是off，这个不用管，purge线程已经停止了
Query OK, 0 rows affected (0.00 sec)
mysql> begin;
mysql> delete from test_purge where b=20;
Query OK, 1 row affected (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.02 sec)
-- 终端会话2
mysql> select * from test_purge;
+---+------+
| a | b | -- 20的那条记录已经删除，但是还没有被purge（purge线程停止）
| 1 | 10 |
| 3 | 30 |
| 4 | 40 |
3 rows in set (0.00 sec)
mysql> insert into test_purge(b) values(20);
Query OK, 1 row affected (0.04 sec)
-- 终端会话3
mysql> show engine innodb status\G
-- ----------------省略其他输出----------------
---TRANSACTION 9497, ACTIVE 19 sec
3 lock struct(s), heap size 1160, 3 row lock(s), undo log entries 1
MySQL thread id 3, OS thread handle 139922002294528, query id 26 localhost root cleaning up
TABLE LOCK table `burn_test`.`test_purge` trx id 9497 lock mode IX
RECORD LOCKS space id 47 page no 4 n bits 72 index b of table `burn_test`.`test_purge` trx id 9497 lock mode S -- S lock （with gap）
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
-- heap no=3表示是第二个插入的记录20
-- 且info bits为32，表示记录被标记删除了
0: len 4; hex 80000014; asc ;; -- 记录为20
1: len 4; hex 80000002; asc ;; -- 对应的主键为2
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
-- heap no=4表示的是20的下一个记录30
-- 且该记录上也有S lock
0: len 4; hex 8000001e; asc ;;
1: len 4; hex 80000003; asc ;;
RECORD LOCKS space id 47 page no 4 n bits 72 index b of table `burn_test`.`test_purge` trx id 9497 lock mode S locks gap before rec
Record lock, heap no 6 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 -- heap no=6为新插入的记录20，从隐式锁提升为显示锁
0: len 4; hex 80000014; asc ;;
1: len 4; hex 80000005; asc ;;

mysql> insert into test_purge(b) values(25);
ERROR 1205 (HY000): Unknown error 1205 -- 等待了一段时间后,超时
---TRANSACTION 9508, ACTIVE 3 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 1160, 1 row lock(s), undo log entries 1
MySQL thread id 5, OS thread handle 139922002560768, query id 46 localhost root update
insert into test_purge(b) values(25) -- 插入的25在等待
------- TRX HAS BEEN WAITING 3 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 47 page no 4 n bits 72 index b of table `burn_test`.`test_purge` trx id 9508 lock_mode X locks gap before rec insert intention waiting
------------------
TABLE LOCK table `burn_test`.`test_purge` trx id 9508 lock mode IX
---TRANSACTION 9503, ACTIVE 10 sec
MySQL thread id 7, OS thread handle 139922002028288, query id 44 localhost root cleaning up
TABLE LOCK table `burn_test`.`test_purge` trx id 9503 lock mode IX
RECORD LOCKS space id 47 page no 4 n bits 72 index b of table `burn_test`.`test_purge` trx id 9503 lock mode S
1: len 4; hex 80000002; asc ;;
RECORD LOCKS space id 47 page no 4 n bits 72 index b of table `burn_test`.`test_purge` trx id 9503 lock mode S locks gap before rec
Record lock, heap no 6 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
1: len 4; hex 80000007; asc ;;