Gathering System Statistics

• System Statistics can be collected and displayed for CBO to use and apprehend CPU and system I/O information. For each plan candidate, the optimizer computes estimates for I/O and CPU costs.
• Invoke the dbms_stats.gather_system_stats procedure as an elapsed time capture, making sure to collect the statistics during a representative heavy workload
• How to collect  these  statistics.easist way  of  it  is
  
  execute dbms_stats.gather_system_stats('Start');
  -- one hour delay during high workload
  execute dbms_stats.gather_system_stats('Stop');
• 
  
   Here are the data items collected by dbms_stats.gather_system_stats: we  can query it  from  aux_stats$ system view
  
  select * from aux_stats$;
  No Workload (NW) stats::

  CPUSPEEDNW - CPU speed

  IOSEEKTIM - The I/O seek time in milliseconds

  IOTFRSPEED - I/O transfer speed in milliseconds

  Workload-related stats:

  SREADTIM - Single block read time in milliseconds

  MREADTIM - Multiblock read time in ms

  CPUSPEED - CPU speed

  MBRC - Average blocks read per multiblock read

  MAXTHR - Maximum I/O throughput

  SLAVETHR - OPQ Factotum (slave)

• If the hardware or workload has changed these  statistics must be  refreshed.
• If  the workload  differs  from day to  nigth  (run OLTP during the day and DSS at night) then  statistics can be  exported  to  a table and  One  job may be   scheculed  to  import  statistics  before  begining  of  night  and   day

See also:
Gather Optimizer Statistics For Sys