MySQL INSERT语句的锁

INSERT…SELECT

表初始化

CREATE TABLE `t` (
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  `c` INT(11) DEFAULT NULL,
  `d` INT(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `c` (`c`)
) ENGINE=InnoDB;

INSERT INTO t VALUES (null,1,1);
INSERT INTO t VALUES (null,2,2);
INSERT INTO t VALUES (null,3,3);
INSERT INTO t VALUES (null,4,4);

CREATE TABLE t2 LIKE t;

操作时序

时刻	SessionA	SessionB
T1		BEGIN;
T2		INSERT INTO t2(c,d) SELECT c,d FROM t;
T3	INSERT INTO t VALUES (-1,-1,-1);(Blocked)

-- T3时刻
mysql> SELECT locked_index,locked_type,waiting_lock_mode,blocking_lock_mode FROM sys.innodb_lock_waits WHERE locked_table='`test`.`t`';
+--------------+-------------+-------------------+--------------------+
| locked_index | locked_type | waiting_lock_mode | blocking_lock_mode |
+--------------+-------------+-------------------+--------------------+
| PRIMARY      | RECORD      | X,GAP             | S                  |
+--------------+-------------+-------------------+--------------------+

1. T2时刻，session B会在表t加上PRIMARY:Next-Key Lock:(-∞,1]
2. 如果没有锁的话，就可能会出现session B的INSERT语句先执行，但对应的binlog后写入的情况
   1. binlog_format=STATEMENT，binlog里面的语句序列如下
      1. INSERT INTO t VALUES (-1,-1,-1)
      2. INSERT INTO t2(c,d) SELECT c,d FROM t
   2. 这个语句传到备库执行，就会把id=-1这一行也会写到t2，主备不一致

INSERT循环写入

非循环写入

mysql> EXPLAIN INSERT INTO t2(c,d) (SELECT c+1,d FROM t FORCE INDEX(c) ORDER BY c DESC LIMIT 1);
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------+
|  1 | INSERT      | t2    | NULL       | ALL   | NULL          | NULL | NULL    | NULL | NULL |     NULL | NULL  |
|  1 | SIMPLE      | t     | NULL       | index | NULL          | c    | 5       | NULL |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------+

# Time: 2019-03-15T04:55:55.315664Z
# User@Host: root[root] @ localhost []  Id:     2
# Query_time: 0.003300  Lock_time: 0.000424 Rows_sent: 0  Rows_examined: 1
SET timestamp=1552625755;
INSERT INTO t2(c,d) (SELECT c+1,d FROM t FORCE INDEX(c) ORDER BY c DESC LIMIT 1);

1. 加锁范围为在表t上c:Next-Key Lock:(3,4]+c:Next-Key Lock:(4,+∞]
2. 执行流程比较简单，从表t中按索引c倒序扫描第一行，拿到结果后写入到表t2，整个语句的扫描行数为1

循环写入

-- MySQL 5.7上执行
mysql> EXPLAIN INSERT INTO t(c,d) (SELECT c+1,d FROM t FORCE INDEX(c) ORDER BY c DESC LIMIT 1);
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
|  1 | INSERT      | t     | NULL       | ALL   | NULL          | NULL | NULL    | NULL | NULL |     NULL | NULL            |
|  1 | SIMPLE      | t     | NULL       | index | NULL          | c    | 5       | NULL |    1 |   100.00 | Using temporary |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-----------------+

mysql> SHOW STATUS LIKE '%Innodb_rows_read%';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| Innodb_rows_read | 13    |
+------------------+-------+

mysql> INSERT INTO t(c,d) (SELECT c+1,d FROM t FORCE INDEX(c) ORDER BY c DESC LIMIT 1);
Query OK, 1 row affected (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> SHOW STATUS LIKE '%Innodb_rows_read%';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| Innodb_rows_read | 17    |
+------------------+-------+

# Time: 2019-03-15T05:10:38.603323Z
# User@Host: root[root] @ localhost []  Id:     2
# Query_time: 0.004470  Lock_time: 0.000184 Rows_sent: 0  Rows_examined: 5
SET timestamp=1552626638;
INSERT INTO t(c,d) (SELECT c+1,d FROM t FORCE INDEX(c) ORDER BY c DESC LIMIT 1);

1. Using temporary表示用到了临时表，执行过程中，需要把表t的内容读出来，写入临时表
2. 实际上，EXPLAIN结果里的rows=1是因为受到了LIMIT 1的影响
3. 语句执行前后，Innodb_rows_read的值增加了4，因为临时表默认使用的是Memory引擎。这4行数据查的是表t，即对表t做了全表扫描
4. 执行流程
   1. 创建临时表，表里有两个字段c和d
   2. 按照索引c扫描表t，依次取出c=4,3,2,1，然后回表，读到c和d的值写入临时表。此时，Rows_examined=4
   3. 由于有LIMIT 1，所以只会取临时表的第一行，再插入到表t。此时，Rows_examined=5
5. 该语句会导致在表t上做全表扫描，并且会给索引c上的所有间隙都加上Share Next-Key Lock。 在这个语句执行期间，其它事务不能在这个表上插入数据
6. 需要临时表的原因
   1. 一边遍历数据，一边更新数据
   2. 如果读出来的数据直接写回原表，可能在遍历过程中，读到刚刚插入的记录
   3. 新插入的记录如果参与计算逻辑，就会与原语义不符

优化方案

CREATE TEMPORARY TABLE temp_t(c INT,d INT) ENGINE=Memory;
-- Rows_examined=1
INSERT INTO temp_t (SELECT c+1, d FROM t FORCE INDEX(c) ORDER BY c DESC LIMIT 1);
-- Rows_examined=1
INSERT INTO t(c,d) SELECT * FROM temp_t;
DROP TABLE temp_t;

INSERT唯一键冲突

时刻	SessionA	SessionB
T0	SELECT * FROM t;
T1	INSERT INTO t VALUES (10,10,10);
T2	BEGIN;
T3	INSERT INTO t VALUES (11,10,10);(Duplicate entry ‘10’ for key ‘c’)
T4		INSERT INTO t VALUES (12,9,9);(Blocked)

-- T0时刻
mysql> SELECT * FROM t;
+----+------+------+
| id | c    | d    |
+----+------+------+
|  1 |    1 |    1 |
|  2 |    2 |    2 |
|  3 |    3 |    3 |
|  4 |    4 |    4 |
|  5 |    5 |    4 |
+----+------+------+

mysql> SELECT locked_index,locked_type,waiting_lock_mode,blocking_lock_mode FROM sys.innodb_lock_waits WHERE locked_table='`test`.`t`';
+--------------+-------------+-------------------+--------------------+
| locked_index | locked_type | waiting_lock_mode | blocking_lock_mode |
+--------------+-------------+-------------------+--------------------+
| c            | RECORD      | X,GAP             | S                  |
+--------------+-------------+-------------------+--------------------+

1. session A要执行的INSERT语句，发生唯一键冲突，并不是简单地报错返回，还需要在冲突的索引上加锁
2. 一个Next-Key Lock由它的右边界定义的，即是c:Shared Next-Key Lock:(5,10]

INSERT死锁

时刻	SessionA	SessionB	SessionC
T0	TRUNCATE t;
T1	BEGIN;INSERT INTO t VALUES (null,5,5);
T2		INSERT INTO t VALUES (null,5,5);	INSERT INTO t VALUES (null,5,5);
T3	ROLLBACK;		Deadlock found

1. 在T1时刻，session A执行INSERT语句，在索引c=5上加上行锁（索引c是唯一索引，可以退化为行锁）
2. 在T2时刻，session B和session C执行相同的INSERT语句，发现唯一键冲突，等待加上读锁
3. 在T3时刻，session A执行ROLLBACK语句，session B和session C都试图继续插入执行操作，都要加上写锁。但两个session都要等待对方的读锁，所以就出现了死锁

INSERT INTO…ON DUPLICATE KEY

TRUNCATE T;
INSERT INTO t VALUES (1,1,1),(2,2,2);

-- 如果有多个列违反唯一性约束，按照索引的顺序，修改跟第一个索引冲突的行
-- 2 rows affected，insert和update都认为自己成功了，update计数加1，insert计数也加1
INSERT INTO t VALUES (2,1,100) ON DUPLICATE KEY UPDATE d=100;
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM t;
+----+------+------+
| id | c    | d    |
+----+------+------+
|  1 |    1 |    1 |
|  2 |    2 |  100 |
+----+------+------+

PS：参考资料 《MySQL实战45讲》