ASH

ASH (Active Session History) is an Oracle Database component that samples the state of every active session once per second and stores the data in an in-memory circular buffer (the V$ACTIVE_SESSION_HISTORY view).

How it works #

Every second Oracle records for each active session:

• Currently executing SQL (SQL_ID)
• Current wait event
• Calling program and module
• Execution plan in use (SQL_PLAN_HASH_VALUE)

Older data is automatically flushed to AWR tables (DBA_HIST_ACTIVE_SESS_HISTORY) and retained for the configured period.

What it’s for #

ASH is the DBA’s microscope: where AWR shows averages over hourly intervals, ASH lets you reconstruct what a single session was doing at a precise moment. It is the ideal tool for:

• Identifying who is running a problematic SQL
• Understanding exactly when a problem started (to the second)
• Correlating sessions, programs and wait events in real time

When to use it #

Use it when the AWR report has already identified a dominant SQL or wait event and you need detail: which session, which program, at what exact time. The rule of thumb: AWR to understand what changed, ASH to understand why.