INSERT...SELECT语句对查询的表加锁吗
前言：insert into t2 select * from t1; 这条语句会对查询表 t1 加锁吗？不要轻易下结论。对GreatSQL的锁进行研究之前，首先要确认一下事务的隔离级别，不同的事务隔离级别，锁的表现是不一样的。
实验：
创建测试表t1，t2
greatsql> create table t1(id int primary key ,c1 varchar(10),c2 datetime,key idx_c1(c1));
greatsql> create table t2 like t1;
# id 列为主键，c1列上有普通索引创建存储过程，向t1表插入测试数据
greatsql> delimiter //
CREATE or replace PROCEDURE p1()
BEGIN
DECLARE p1 int default 0;
while p1<5 do
insert into t1(id,c1,c2) values(p1*2,round(rand()*10000),now());
SET p1 = p1 + 1;
end while;
END;
//
delimiter ;
greatsql> call p1;
greatsql> select * from t1;
+----+------+---------------------+
| id | c1 | c2 |
+----+------+---------------------+
|0 | 2660 | 2024-02-21 15:45:00 |
|2 | 4627 | 2024-02-21 15:45:00 |
|4 | 5158 | 2024-02-21 15:45:00 |
|6 | 1907 | 2024-02-21 15:45:00 |
|8 | 4061 | 2024-02-21 15:45:00 |
+----+------+---------------------+
5 rows in set (0.01 sec)REPEATABLE-READ隔离级别：
查询当前事务隔离级别：
greatsql> show variables like 'transaction_isolation';
+-----------------------+-----------------+
| Variable_name | Value |
+-----------------------+-----------------+
| transaction_isolation | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)connection 1：
greatsql> select ps_current_thread_id();
+------------------------+
| ps_current_thread_id() |
+------------------------+
| 92 |
+------------------------+
1 row in set (0.00 sec)
greatsql> begin;
Query OK, 0 rows affected (0.00 sec)
greatsql> insert into t2 select * from t1;
Query OK, 5 rows affected (0.00 sec)
Records: 5Duplicates: 0Warnings: 0connection2：
greatsql> select ps_current_thread_id();
+------------------------+
| ps_current_thread_id() |
+------------------------+
| 93 |
+------------------------+
1 row in set (0.00 sec)
greatsql> begin;
Query OK, 0 rows affected (0.01 sec)
greatsql> insert into t1(id,c1) values(1,'a');connection3：
greatsql> select THREAD_ID,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from data_locks;
+-----------+-------------+------------+-----------+------------------------+-------------+------------------------+
| THREAD_ID | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+-------------+------------+-----------+------------------------+-------------+------------------------+
| 93 | t1 | NULL | TABLE | IX | GRANTED | NULL |
| 93 | t1 | PRIMARY | RECORD | X,GAP,INSERT_INTENTION | WAITING | 2 |
| 92 | t2 | NULL | TABLE | IX | GRANTED | NULL |
| 92 | t1 | NULL | TABLE | IS | GRANTED | NULL |
| 92 | t1 | PRIMARY | RECORD | S | GRANTED | supremum pseudo-record |
| 92 | t1 | PRIMARY | RECORD | S | GRANTED | 0 |
| 92 | t1 | PRIMARY | RECORD | S | GRANTED | 2 |
| 92 | t1 | PRIMARY | RECORD | S | GRANTED | 4 |
| 92 | t1 | PRIMARY | RECORD | S | GRANTED | 6 |
| 92 | t1 | PRIMARY | RECORD | S | GRANTED | 8 |
+-----------+-------------+------------+-----------+------------------------+-------------+------------------------+
10 rows in set (0.00 sec)connection1的语句中select的表t1上每条记录及最大伪记录supremum pseudo-record都加了S锁，这个S锁是nextkey lock锁，当connection2试图向t1表中插入一条表中不存在的数据时也会被阻塞，connect1的S锁与connect2需要的 X,GAP,INSERT_INTENTION锁不兼容。
在 REPEATABLE-READ 隔离级别下，INSERT ... SELECT 操作并未采用MVCC来保证事务一致性和隔离性，而是使用了锁机制。
加锁的目的是确保事务在读取数据时能够看到一个一致的数据快照。如果在执行 INSERT ... SELECT 时不加锁，那么可能会出现以下情况：
[*]不可重复读：如果在 INSERT ... SELECT 执行期间，另一个事务修改了被查询的数据，那么 INSERT ... SELECT 可能会读取到不同的数据，导致插入的数据不一致。
[*]幻读：在某些情况下，另一个事务可能会在 INSERT ... SELECT 执行期间插入新的行，导致插入操作插入到不应该插入的行。
通过加锁，InnoDB 能够确保 INSERT ... SELECT 语句在执行期间读取到的数据是一致的，并且不会被其他事务修改，从而维护了事务的隔离性和一致性。尽管 MVCC 可以在大多数情况下提供高效的数据读取和写入，但它并不能完全替代锁机制。在 INSERT ... SELECT 这样的操作中，使用 MVCC 可能无法提供足够的保证。
READ-COMMITTED隔离级别
查询当前事务隔离级别：
greatsql> show variables like 'transaction_isolation';
+-----------------------+----------------+
| Variable_name | Value |
+-----------------------+----------------+
| transaction_isolation | READ-COMMITTED |
+-----------------------+----------------+
1 row in set (0.00 sec)connection 1
greatsql> begin;
Query OK, 0 rows affected (0.00 sec)
greatsql> insert into t2 select * from t1;
Query OK, 5 rows affected (0.01 sec)
Records: 5Duplicates: 0Warnings: 0connection 2
greatsql> begin;
Query OK, 0 rows affected (0.00 sec)
greatsql> insert into t1(id,c1) values(1,'a');
Query OK, 1 row affected (0.00 sec)connection3
greatsql> select THREAD_ID,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from data_locks;
+-----------+-------------+------------+-----------+-----------+-------------+-----------+
| THREAD_ID | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+-------------+------------+-----------+-----------+-------------+-----------+
| 104 | t1 | NULL | TABLE | IX | GRANTED | NULL |
| 103 | t2 | NULL | TABLE | IX | GRANTED | NULL |
+-----------+-------------+------------+-----------+-----------+-------------+-----------+
2 rows in set (0.00 sec)可以看出事务隔离级别设置为READ-COMMITTED后，表现截然不同。connection2并没有被阻塞，两个会话持有的锁都只有插入表意向排他锁(IX)。
结论：
INSERT...SELECT语句是否对查询表加锁跟事务隔离级别有关，REPEATABLE-READ隔离级别下加共享读锁，此共享读锁属于Nextkey lock，会影响其他事务对查询表的DML操作；READ-COMMITTED下不加锁，不影响其他事务对表进行DML操作。
Enjoy GreatSQL
来源:https://www.cnblogs.com/greatsql/p/18055878
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