Measure Number of Active Sessions

--  measure number of Active Sessions

select
   round(sample_time,'MI'),
   count(*) as active_sessions
from
   dba_hist_active_sess_history
where
   sample_time > sysdate - 7
group by
   round(sample_time, 'MI');

Find Transactions Per Second (TPS) in Oracle

Use following query to find TPS (overall) in Oracle.

WITH hist_snaps
     AS (SELECT instance_number,
                snap_id,
                round(begin_interval_time,'MI') datetime,
                  (  begin_interval_time
                   + 0
                   - LAG (
                        begin_interval_time + 0)
                     OVER (PARTITION BY dbid, instance_number
                           ORDER BY snap_id))
                * 86400      -- seconds per day
                   diff_time -- calculate difference in time from
                             -- previous snapshot
                   FROM dba_hist_snapshot),
     hist_stats      AS (SELECT dbid,
                instance_number,
                snap_id,
                stat_name,
                  VALUE
                - LAG (
                     VALUE)
                  OVER (PARTITION BY dbid,instance_number,stat_name
                        ORDER BY snap_id)
                  delta_value -- difference in value from previous
                              -- snapshot for each stat_name
           FROM dba_hist_sysstat
          WHERE stat_name IN ('user commits', 'user rollbacks'))
  SELECT datetime,
         ROUND (SUM (delta_value) / 3600, 2) "Transactions/Second"
     FROM hist_snaps sn, hist_stats st
   WHERE     st.instance_number = sn.instance_number
         AND st.snap_id = sn.snap_id
         AND diff_time IS NOT NULL
GROUP BY datetime
ORDER BY 1 desc;

Changing Database link password without recreating

 It is possible to change a database link password without dropping and recreating a database link.

The ALTER DATABASE LINK statement can be used and you do not need to specify the target TNS Service name either.

ALTER DATABASE LINK DB_LINK_NAME connect to USER_NAME identified by  NEW_PASSWORD;

main url: https://jhdba.wordpress.com/2015/02/13/changing-a-database-link-password/