SET @start_global_value = @@global.extra_max_connections;
select @@global.extra_max_connections;
@@global.extra_max_connections
1
select @@session.extra_max_connections;
ERROR HY000: Variable 'extra_max_connections' is a GLOBAL variable
show global variables like 'extra_max_connections';
Variable_name	Value
extra_max_connections	1
show session variables like 'extra_max_connections';
Variable_name	Value
extra_max_connections	1
select * from information_schema.global_variables where variable_name='extra_max_connections';
VARIABLE_NAME	VARIABLE_VALUE
EXTRA_MAX_CONNECTIONS	1
select * from information_schema.session_variables where variable_name='extra_max_connections';
VARIABLE_NAME	VARIABLE_VALUE
EXTRA_MAX_CONNECTIONS	1
set global extra_max_connections=1;
select @@global.extra_max_connections;
@@global.extra_max_connections
1
set session extra_max_connections=1;
ERROR HY000: Variable 'extra_max_connections' is a GLOBAL variable and should be set with SET GLOBAL
set global extra_max_connections=1.1;
ERROR 42000: Incorrect argument type to variable 'extra_max_connections'
set global extra_max_connections=1e1;
ERROR 42000: Incorrect argument type to variable 'extra_max_connections'
set global extra_max_connections="foo";
ERROR 42000: Incorrect argument type to variable 'extra_max_connections'
set global extra_max_connections=0;
Warnings:
Warning	1292	Truncated incorrect extra_max_connections value: '0'
select @@global.extra_max_connections;
@@global.extra_max_connections
1
set global extra_max_connections=cast(-1 as unsigned int);
Warnings:
Note	1105	Cast to unsigned converted negative integer to it's positive complement
Warning	1292	Truncated incorrect extra_max_connections value: '18446744073709551615'
select @@global.extra_max_connections;
@@global.extra_max_connections
100000
SET @@global.extra_max_connections = @start_global_value;