set debug= 'T';
select @@debug;
@@debug
T
set debug= '+P';
select @@debug;
@@debug
P:T
set debug= '-P';
select @@debug;
@@debug
T
set session debug="t";
show session variables like 'debug';
Variable_name	Value
debug	t
set session debug="t";
show session variables like 'debug';
Variable_name	Value
debug	t
set session debug="d:t";
show session variables like 'debug';
Variable_name	Value
debug	d:t
show session variables like 'debug';
Variable_name	Value
debug	t