MySQL Bugs: #119407: Isolation Level Settings

Bug #119407	Isolation Level Settings
Submitted:	17 Nov 2025 6:17	Modified:	19 Nov 2025 4:15
Reporter:	Trevor Trevor	Email Updates:
Status:	Open	Impact on me:	None
Category:	MySQL Server: Locking	Severity:	S3 (Non-critical)
Version:	8.0.41	OS:	Any
Assigned to:		CPU Architecture:	Any
Tags:	Isolation level

Description:
Executing the following SQL statement should change the session-level isolation level, but in reality, the isolation level has not changed.
sql：set @@transaction_isolation= SERIALIZABLE;

How to repeat:
mysql> select @@transaction_isolation;
+-------------------------+
| @@transaction_isolation |
+-------------------------+
| REPEATABLE-READ         |
+-------------------------+
1 row in set (0.00 sec)

mysql> set @@transaction_isolation= SERIALIZABLE;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@transaction_isolation;
+-------------------------+
| @@transaction_isolation |
+-------------------------+
| REPEATABLE-READ         |
+-------------------------+
1 row in set (0.00 sec)

Found the cause.
set @@transaction_isolation equals set TRANSACTION ISOLATION LEVEL，without key word global or session，the scope is on the next trx.

https://dev.mysql.com/doc/refman/8.0/en/set-transaction.html
Without any SESSION or GLOBAL keyword:

The statement applies only to the next single transaction performed within the session.

Subsequent transactions revert to using the session value of the named characteristics.

The statement is not permitted within transactions: