We have all been caught out with using SET and not realizing that the default GLOBAL Scope (since 5.0.2) does not change the current SESSION scope.

I was not aware until today that changing GLOBAL scope has some exceptions that also automatically affect SESSION scope.

What I expected with a change in the GLOBAL scope is no affect SESSION scope. For example.

mysql> SHOW GLOBAL VARIABLES LIKE 'read_buffer_size';
+------------------+--------+
| Variable_name    | Value  |
+------------------+--------+
| read_buffer_size | 131072 |
+------------------+--------+
1 row in set (0.00 sec)

mysql> SHOW SESSION VARIABLES LIKE 'read_buffer_size';
+------------------+--------+
| Variable_name    | Value  |
+------------------+--------+
| read_buffer_size | 131072 |
+------------------+--------+
1 row in set (0.00 sec)

mysql> SET GLOBAL read_buffer_size=1024*256;
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW GLOBAL VARIABLES LIKE …