AWR captures a variety of statistics.
AWR stores base statistics, that is, counters and value statistics (for
example, log file switches and process memory allocated). AWR captures SQL
statistics such as disk reads per SQL statement. Metrics such as physical reads
per minute are also captured.
The Active Session History (ASH) data
is captured first to memory at one-second intervals for only those sessions
that are currently active (performing a database call). Then the ASH data is
reduced by a factor of ten by storing to disk a random sample of the in-memory
data. The ASH data is heavily used by Automatic Database Diagnostic Monitor
(ADDM) to identify root causes of performance issues.
The advisor reports produced by ADDM,
the Segment Advisor, and other advisors are also stored in AWR for later
viewing.
The statistics exist at two levels: the
in-memory recent statistics in V$ views, and persistent
statistics that are stored on disk as snapshots and DBA_* views.
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