select @@global.plugin_dir;
@@global.plugin_dir
MYSQL_TMP_DIR/
select @@session.plugin_dir;
ERROR HY000: Variable 'plugin_dir' is a GLOBAL variable
show global variables like 'plugin_dir';
Variable_name	Value
plugin_dir	MYSQL_TMP_DIR/
show session variables like 'plugin_dir';
Variable_name	Value
plugin_dir	MYSQL_TMP_DIR/
select * from information_schema.global_variables where variable_name='plugin_dir';
VARIABLE_NAME	VARIABLE_VALUE
PLUGIN_DIR	MYSQL_TMP_DIR/
select * from information_schema.session_variables where variable_name='plugin_dir';
VARIABLE_NAME	VARIABLE_VALUE
PLUGIN_DIR	MYSQL_TMP_DIR/
set global plugin_dir=1;
ERROR HY000: Variable 'plugin_dir' is a read only variable
set session plugin_dir=1;
ERROR HY000: Variable 'plugin_dir' is a read only variable
