Active Session History (ASH) records very recent session activity by collecting Active Session History statistics (mostly the wait statistics for different events) for all active sessions every second, and stores them in a circular buffer in the SGA.
The V$SESSION view holds all the session data for all current sessions. ASH samples the V$SESSION view and gets the most critical wait information from it.
The V$ACTIVE_SESSION_HISTORY view is where the database stores a sample of all active session data.
The ashrpt.sql script located in the $ORACLE_HOME/rdbms/admin directory generates information about the SQL that ran during the time you specify, and it includes blocking and wait details.
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