对于没有索引的表，MySQL会使用表级锁，写操作不会阻塞读操作，读操作不会阻塞写操作；一个会话的写操作会对整张表加锁，其他会话想修改表需要等到这个会话提交或回滚事务。

会话①
mysql> create table t12(id tinyint(3) unsigned not null,
->name varchar(10) not null)
->engine=innodb auto_increment=8 default charset=gbk;
Query OK, 0 rows affected (0.12 sec)

mysql> show keys from t12;
Empty set (0.00 sec)

mysql> show variables like '%commit%';
+--------------------------------+-------+
| Variable_name | Value |
+--------------------------------+-------+
| autocommit | ON|
| innodb_commit_concurrency | 0 |
| innodb_flush_log_at_trx_commit | 1 |
+--------------------------------+-------+
3 rows in set (0.00 sec)

mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like '%commit%';
+--------------------------------+-------+
| Variable_name | Value |
+--------------------------------+-------+
| autocommit | OFF|
| innodb_commit_concurrency | 0 |
| innodb_flush_log_at_trx_commit | 1 |
+--------------------------------+-------+
3 rows in set (0.00 sec)

mysql> insert into t12 values(10,'Neo');
Query OK, 1 row affected (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from t12;
+----+------+
| id | name |
+----+------+
| 10 | Neo |
+----+------+
1 row in set (0.00 sec)

会话②
mysql> show variables like '%commit%';
+--------------------------------+-------+
| Variable_name | Value |
+--------------------------------+-------+
| autocommit | ON|
| innodb_commit_concurrency | 0 |
| innodb_flush_log_at_trx_commit | 1 |
+--------------------------------+-------+
3 rows in set (0.00 sec)

mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like '%commit%';
+--------------------------------+-------+
| Variable_name | Value |
+--------------------------------+-------+
| autocommit | OFF|
| innodb_commit_concurrency | 0 |
| innodb_flush_log_at_trx_commit | 1 |
+--------------------------------+-------+
3 rows in set (0.00 sec)

mysql> begin;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from t12;
+----+------+
| id | name |
+----+------+
| 10 | Neo |
+----+------+
1 row in set (0.00 sec)

会话①
mysql> update t12 set name='trinity' where id=10;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1 Changed: 1 Warnings: 0

会话②
下面DML语句会一直阻塞
mysql> insert into t12 values(20,'Trinity');

过一段时间会出现超时提示
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

会话①
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)

会话②
mysql> insert into t12 values(20,'Trinity');
Query OK, 1 row affected (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.01 sec)

对于有索引的表，写操作不会阻塞读操作，读操作不会阻塞写操作；如果在MySQL在写操作时使用索引扫描，则会使用行级锁，一个会话的写操作会对修改的行加锁，其他会话想修改这些行需要等到这个会话提交或回滚事务，其他会话对其他行的写操作不受影响，行锁会阻塞表锁；如果MySQL使用全表扫描，则会使用表级锁，一个会话的写操作会对整张表加锁，其他会话想修改表需要等到这个会话提交或回滚事务，表锁会阻塞行锁。

会话①
mysql> create index idx_t12_id on t12(id);
Query OK, 0 rows affected (0.26 sec)
Records: 0 Duplicates: 0 Warnings: 0

查看索引
mysql> show keys from t12;
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name| Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t12| 1 | idx_t12_id |1 | id | A |3 | NULL | NULL| | BTREE | ||
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

查看执行计划
mysql> explain select * from t12 where id=20;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra|
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t12| ALL | idx_t12_id| NULL | NULL| NULL |1 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

mysql> delete from t12 where id=20;
Query OK, 2 rows affected (0.00 sec)

mysql> select * from t12;
+----+------+
| id | name |
+----+------+
| 10 | Neo |
+----+------+
1 row in set (0.00 sec)

会话②

查看执行计划
mysql> explain select * from t12 where id=10;
+----+-------------+-------+------+---------------+------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key| key_len | ref| rows | Extra |
+----+-------------+-------+------+---------------+------------+---------+-------+------+-------+
| 1 | SIMPLE | t12| ref | idx_t12_id| idx_t12_id | 1| const |1 ||
+----+-------------+-------+------+---------------+------------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> update t12 set name='Jack' where id=10;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

mysql> select * from t12;
+----+---------+
| id | name|
+----+---------+
| 10 | Jack|
| 20 | Trinity |
| 20 | Trinity |
+----+---------+
3 rows in set (0.00 sec)

mysql> rollback;
Query OK, 0 rows affected (0.01 sec)

mysql> show processlist;
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+
| Id | User| Host | db| Command | Time| State| Info |
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+
| 1 | system user || NULL | Connect | 769140 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL |
| 2 | system user || NULL | Connect | 769141 | Connecting to master| NULL |
| 13 | event_scheduler | localhost | NULL | Daemon | 621090 | Waiting on empty queue | NULL |
| 76 | neo | localhost | fire | Sleep|180 | | NULL |
| 78 | neo | localhost | fire | Query| 0 | NULL| show processlist |
+----+-----------------+-----------+------+---------+--------+-----------------------------------------------------------------------------+------------------+
5 rows in set (0.00 sec)

mysql> explain select * from t12;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | t12| ALL | NULL | NULL | NULL| NULL |1 ||
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

下面的更新没有使用索引而使用全表扫描，这样会加表级锁，会处于阻塞状态。
mysql> update t12 set name='Jack';
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

会话①

mysql> rollback;
Query OK, 0 rows affected (0.05 sec)

会话②
随着会话①的回滚操作，会话②执行成功
mysql> update t12 set name='Jack';
Query OK, 3 rows affected (12.41 sec)
Rows matched: 3 Changed: 3 Warnings: 0

mysql> rollback;
Query OK, 0 rows affected (0.00 sec)

会话①
mysql> update t12 set name='Jack';
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3 Changed: 3 Warnings: 0

会话②
插入操作会一直处于阻塞状态
mysql> insert into t12 values(30,'Lily');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

会话①
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)

会话②
mysql> insert into t12 values(30,'Lily');
Query OK, 1 row affected (0.09 sec)

两行数据使用了同一个索引，对两个不同的行加锁，也会引起锁等待
mysql> show create table tab_with_index\G
*************************** 1. row ***************************
Table: tab_with_index
Create Table: CREATE TABLE `tab_with_index` (
`id` int(11) DEFAULT NULL,
`name` varchar(10) DEFAULT NULL,
KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> select * from tab_with_index where id=1;
+------+------+
| id | name |
+------+------+
| 1 | 1 |
| 1 | 4 |
+------+------+
2 rows in set (0.00 sec)

mysql> show keys from tab_with_index;
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tab_with_index | 1 | id | 1 | id | A | 7 | NULL | NULL | YES | BTREE | | |
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

会话①
使用where id=1 and name='1'条件进行查询
mysql> select * from tab_with_index where id=1 and name='1' for update;
+------+------+
| id | name |
+------+------+
| 1 | 1 |
+------+------+
1 row in set (0.00 sec)

会话②
查询where id=1 and name='4'条件进行查询，由于和会话①使用了相同的索引，即使查询了不同的字段，也会引起锁等待
mysql> select * from tab_with_index where id=1 and name='4' for update;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

在这种情况下，可以考虑创建联合索引
会话①
mysql> create index idx_id_name on tab_with_index(id,name);
Query OK, 0 rows affected (0.24 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> select * from tab_with_index where id=1 and name='4' for update;
+------+------+
| id | name |
+------+------+
| 1 | 4 |
+------+------+
1 row in set (0.00 sec)

会话②
mysql> select * from tab_with_index where id=1 and name='1' for update;
+------+------+
| id | name |
+------+------+
| 1 | 1 |
+------+------+
1 row in set (0.00 sec)

InnoDB存储引擎的表使用不同索引的阻塞例子

会话①
mysql> show create table tab_with_index\G
*************************** 1. row ***************************
Table: tab_with_index
Create Table: CREATE TABLE `tab_with_index` (
`id` int(11) DEFAULT NULL,
`name` varchar(10) DEFAULT NULL,
KEY `id` (`id`),
KEY `idx_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> desc tab_with_index;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | MUL | NULL | |
| name | varchar(10) | YES | MUL | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> select * from tab_with_index;
+------+------+
| id | name |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
| 1 | 4 |
+------+------+
5 rows in set (0.00 sec)

mysql> explain select * from tab_with_index where id=1 for update;
+----+-------------+----------------+------+---------------+------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------+------+---------------+------+---------+-------+------+-------+
| 1 | SIMPLE | tab_with_index | ref | id | id | 5 | const | 2 | NULL |
+----+-------------+----------------+------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> select * from tab_with_index where id=1;
+------+------+
| id | name |
+------+------+
| 1 | 1 |
| 1 | 4 |
+------+------+
2 rows in set (0.00 sec)

mysql> select * from tab_with_index where id=1 for update;
+------+------+
| id | name |
+------+------+
| 1 | 1 |
| 1 | 4 |
+------+------+
2 rows in set (0.01 sec)

会话②
mysql> explain select * from tab_with_index where name='4' for update;
+----+-------------+----------------+------+---------------+----------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------+------+---------------+----------+---------+-------+------+-----------------------+
| 1 | SIMPLE | tab_with_index | ref | idx_name | idx_name | 13 | const | 2 | Using index condition |
+----+-------------+----------------+------+---------------+----------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)

mysql> select * from tab_with_index where name='4' for update;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction