Oracle Performance & Tuning

Check current PGA and SGA :

show parameter sga;
show parameter pga;

Show current and maximum sessions :

SELECT
  'Currently, ' 
  || (SELECT COUNT(*) FROM V$SESSION)
  || ' out of ' 
  || VP.VALUE 
  || ' connections are used.' AS USAGE_MESSAGE
FROM 
  V$PARAMETER VP
WHERE VP.NAME = 'sessions';

AWR Retention period :

select snap_interval, retention from dba_hist_wr_control;