Automatic Monitoring and Alerting

In oracle 10g we were introduced MMON process which collects metrics and
statistics.
No more complex queries for monitoring the database
No more heavy load on the database done by our own monitoring scripts
Just query one view or two.
Of course that you can enhance this mechanism by changing the thresholds,
push the alerts via mail...
All of these can be done via SQL or by using Enterprise Manager/Database
Console.

Warning & critical threshold can be set by using DBMS_SERVER_ALERT package

Thresholds definition can reviewed by using:
SELECT object_name, metrics_name, warning_value, critical_value
FROM dba_thresholds;

Outstanding alerts can be reviewed by using:
SELECT reason FROM dba_outstanding_alerts;

When the alerts are cleared it can be reviewed by querying DBA_ALERT_HISTORY

See: Using DBMS_SERVER_ALERT