mysql: deleted performance_schema, is it a problem?
The MySQL Performance Schema is a feature for monitoring MySQL Server execution at a low level. The Performance Schema is available as of MySQL 5.5.3.

From MySQL Manual there are some of the features of the performance_schema database.


  The Performance Schema provides a way to inspect internal execution of
  the server at runtime. It is implemented using the PERFORMANCE_SCHEMA
  storage engine and the performance_schema database. The Performance
  Schema focuses primarily on performance data. This differs from
  INFORMATION_SCHEMA, which serves for inspection of metadata.
  
  The Performance Schema monitors server events. An “event” is anything
  the server does that takes time and has been instrumented so that
  timing information can be collected. In general, an event could be a
  function call, a wait for the operating system, a stage of an SQL
  statement execution such as parsing or sorting, or an entire statement
  or group of statements. Currently, event collection provides access to
  information about synchronization calls (such as for mutexes) file and
  table I/O, table locks, and so forth for the server and for several
  storage engines
  
  Performance Schema events are distinct from events written to the
  server's binary log (which describe data modifications) and Event
  Scheduler events (which are a type of stored program).


For detailed information look this Link Performance Schema.