1select @@global.extra_port; 2@@global.extra_port 30 4select @@session.extra_port; 5ERROR HY000: Variable 'extra_port' is a GLOBAL variable 6show global variables like 'extra_port'; 7Variable_name Value 8extra_port 0 9show session variables like 'extra_port'; 10Variable_name Value 11extra_port 0 12select * from information_schema.global_variables where variable_name='extra_port'; 13VARIABLE_NAME VARIABLE_VALUE 14EXTRA_PORT 0 15select * from information_schema.session_variables where variable_name='extra_port'; 16VARIABLE_NAME VARIABLE_VALUE 17EXTRA_PORT 0 18set global extra_port=1; 19ERROR HY000: Variable 'extra_port' is a read only variable 20set session extra_port=1; 21ERROR HY000: Variable 'extra_port' is a read only variable 22