root@localhost : test 10:56:10>create table t(a int,key idx_a(a))engine =innodb;
Query OK, 0 rows affected (0.20 sec)

root@localhost : test 10:56:13>insert into t values(1),(3),(5),(8),(11);
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

root@localhost : test 10:56:15>select
 
 * from t;
+------+
| a    |
+------+
|    1 |
|    3 |
|    5 |
|    8 |
|   11 |
+------+
5 rows in set (0.00 sec)

section A:

root@localhost : test 10:56:27>start transaction;
Query OK, 0 rows affected (0.00 sec)

root@localhost : test 10:56:29>select * from t where a = 8 for update;
+------+
| a    |
 

+------+
|    8 |
+------+
1 row in set (0.00 sec)


section B:
root@localhost : test 10:54:50>begin;
Query OK, 0 rows affected (0.00 sec)

root@localhost : test 10:56:51>select * from t;
+------+
| a    |
+------+
|    1 |
|    3 |
|    5 |
|    8 |
|   11 |
+------+
5 rows in set (0.00 sec)

root@localhost
 
 : test 10:56:54>insert into t values(2);
Query OK, 1 row affected (0.00 sec)

root@localhost : test 10:57:01>insert into t values(4);
Query OK, 1 row affected (0.00 sec)

++++++++++
root@localhost : test 10:57:04>insert into t values(6);

root@localhost : test 10:57:11>insert into t values(7);

root@localhost : test 10:57:15>insert into t values(9);

root@localhost : test 10:57:33>insert into t values(10);
++++++++++
上面全被鎖住，阻塞住了

root@localhost : test 10:57:39>insert into t values(12);
Query OK, 1 row affected (0.00 sec)