text: select @@global.performance_schema_max_file_classes;
  @@global.performance_schema_max_file_classes
  123
  select @@session.performance_schema_max_file_classes;
  ERROR HY000: Variable 'performance_schema_max_file_classes' is a GLOBAL variable
  show global variables like 'performance_schema_max_file_classes';
  Variable_name	Value
  performance_schema_max_file_classes	123
  show session variables like 'performance_schema_max_file_classes';
  Variable_name	Value
  performance_schema_max_file_classes	123
  select * from information_schema.global_variables
  where variable_name='performance_schema_max_file_classes';
  VARIABLE_NAME	VARIABLE_VALUE
  PERFORMANCE_SCHEMA_MAX_FILE_CLASSES	123
  select * from information_schema.session_variables
  where variable_name='performance_schema_max_file_classes';
  VARIABLE_NAME	VARIABLE_VALUE
  PERFORMANCE_SCHEMA_MAX_FILE_CLASSES	123
  set global performance_schema_max_file_classes=1;
  ERROR HY000: Variable 'performance_schema_max_file_classes' is a read only variable
  set session performance_schema_max_file_classes=1;
  ERROR HY000: Variable 'performance_schema_max_file_classes' is a read only variable
  