Active Session History
The Active Session History (ASH) contains
recent session activity. The memory assigned to the ASH comes
from the SGA and is fixed for the instance lifetime. Its size
also cannot exceed 5% of the shared pool size and takes up 2MB
per CPU within these limits.
The ASH works by sampling the v$session
view every second and recording
events the sessions are waiting for. The sampling process
accesses the database internal structures directly. During the
sampling process, inactive sessions are ignored. The ASH is
designed as a rolling buffer in memory by over-writing older
information as needed. The v$active_session_history view
is used to access the ASH statistics. This view will contain one
row for every active session per sample.