1、鎖的類型分為讀鎖和寫鎖，這個很好區分。可以這樣認為：如果有增刪改，就是寫鎖。如果是查詢，就是讀鎖。
2、鎖的粒度也就是鎖的范圍，分為行鎖和表鎖。鎖的范圍和多個因素有關，包括事務隔離級別、是否使用索引。
測試 read-committed，結果是行鎖
事務A:
mysql> select @@session.tx_isolation;
+------------------------+
| @@session.tx_isolation |
+------------------------+
| READ-COMMITTED |
+------------------------+
1 row in set (0.00 sec)
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set num=num+1 where id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
事務B:
mysql> select @@session.tx_isolation;
+------------------------+
| @@session.tx_isolation |
+------------------------+
| READ-COMMITTED |
+------------------------+
1 row in set (0.00 sec)
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set num=num+1 where id=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
查詢事務:
mysql> select * from information_schema.innodb_trx\G
*************************** 1. row ***************************
trx_id: DAB09
trx_state: RUNNING
trx_started: 2015-07-02 08:41:28
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 3
trx_mysql_thread_id: 3
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 0
trx_lock_structs: 2
trx_lock_memory_bytes: 320
trx_rows_locked: 1
trx_rows_modified: 1
trx_concurrency_tickets: 0
trx_isolation_level: READ COMMITTED
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 10000
*************************** 2. row ***************************
trx_id: DAB08
trx_state: RUNNING
trx_started: 2015-07-02 08:41:00
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 3
trx_mysql_thread_id: 2
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 0
trx_lock_structs: 2
trx_lock_memory_bytes: 320
trx_rows_locked: 1
trx_rows_modified: 1
trx_concurrency_tickets: 0
trx_isolation_level: READ COMMITTED
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 10000
2 rows in set (0.00 sec)
兩個事務之間沒有lock wait，說明read-committed是行鎖
測試 repeatable-read，結果是表鎖
事務A:
mysql> select @@session.tx_isolation;
+------------------------+
| @@session.tx_isolation |
+------------------------+
| REPEATABLE-READ |
+------------------------+
1 row in set (0.01 sec)
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set num=num+1 where id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
事務B:
mysql> select @@session.tx_isolation;
+------------------------+
| @@session.tx_isolation |
+------------------------+
| REPEATABLE-READ |
+------------------------+
1 row in set (0.00 sec)
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set num=num+1 where id=2;
查詢事務:
mysql> select * from information_schema.innodb_trx\G
*************************** 1. row ***************************
trx_id: DAB1A
trx_state: LOCK WAIT
trx_started: 2015-07-02 09:31:29
trx_requested_lock_id: DAB1A:0:100841:2
trx_wait_started: 2015-07-02 09:31:29
trx_weight: 2
trx_mysql_thread_id: 3
trx_query: update test set num=num+1 where id=2
trx_operation_state: starting index read
trx_tables_in_use: 1
trx_tables_locked: 1
trx_lock_structs: 2
trx_lock_memory_bytes: 320
trx_rows_locked: 1
trx_rows_modified: 0
trx_concurrency_tickets: 0
trx_isolation_level: REPEATABLE READ
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 10000
*************************** 2. row ***************************
trx_id: DAB19
trx_state: RUNNING
trx_started: 2015-07-02 09:31:21
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 3
trx_mysql_thread_id: 2
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 0
trx_lock_structs: 2
trx_lock_memory_bytes: 320
trx_rows_locked: 4
trx_rows_modified: 1
trx_concurrency_tickets: 0
trx_isolation_level: REPEATABLE READ
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 10000
2 rows in set (0.00 sec)
事務B處於lock wait狀態，說明repeatable-read是表鎖
測試 repeatable-read，查詢條件使用主鍵，也就是id為primary key，結果是行鎖
mysql> desc test;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| ID | int(11) | NO | | 0 | |
| NUM | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql> alter table test add primary key(id);
Query OK, 0 rows affected (14.70 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc test;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| ID | int(11) | NO | PRI | 0 | |
| NUM | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.01 sec)
事務A:
mysql> select @@session.tx_isolation;
+------------------------+
| @@session.tx_isolation |
+------------------------+
| REPEATABLE-READ |
+------------------------+
1 row in set (0.00 sec)
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set num=num+1 where id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
事務B:
mysql> select @@session.tx_isolation;
+------------------------+
| @@session.tx_isolation |
+------------------------+
| REPEATABLE-READ |
+------------------------+
1 row in set (0.00 sec)
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set num=num+1 where id=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
查詢事務:
mysql> select * from information_schema.innodb_trx\G
*************************** 1. row ***************************
trx_id: DAB23
trx_state: RUNNING
trx_started: 2015-07-02 09:38:48
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 3
trx_mysql_thread_id: 3
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 0
trx_lock_structs: 2
trx_lock_memory_bytes: 320
trx_rows_locked: 1
trx_rows_modified: 1
trx_concurrency_tickets: 0
trx_isolation_level: REPEATABLE READ
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 10000
*************************** 2. row ***************************
trx_id: DAB22
trx_state: RUNNING
trx_started: 2015-07-02 09:38:45
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 3
trx_mysql_thread_id: 2
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 0
trx_lock_structs: 2
trx_lock_memory_bytes: 320
trx_rows_locked: 1
trx_rows_modified: 1
trx_concurrency_tickets: 0
trx_isolation_level: REPEATABLE READ
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 10000
2 rows in set (0.00 sec)
表test的id為主鍵，上面的兩個事務，沒有lock wait，說明通過主鍵操作，是行鎖。為什么？
這是因為innodb主鍵索引（聚集索引），可以直接定位相應的行，不需要鎖住整個表。對於輔助索引也是同樣的道理，是行鎖。因為對於輔助索引，也要使用主鍵索引定位到相應的行。
也就是說innodb 使用索引，只會去鎖相應的行（有可能還包括當前行附近的行），而不是鎖住整個表。
注意：查詢加鎖信息，除了表innodb_trx，還有表innodb_locks, innodb_lock_waits，這些表都在information_schema數據庫