Monday, 25 January 2016

What's Going On? Oracle Activity Monitoring Views

How do you find out what is happening now inside an Oracle database instance when you only have SQL level access to the database instance, and no nice GUI management tool? I'll cover how to start doing this with some SQL query examples.

A user is complaining that the application is running slowly, or a report someone scheduled is taking too long to finish - where do you look to find out what is going on inside the Oracle database right now? Well if you have a nice GUI based tool such as Enterprise Manager then go there first, because that will have summary screens of what is going on, and also drill downs into the details behind what is going on. Such tools are easier and quicker to use, and mean you don't have to remember the names of internal Oracle dynamic performance views.

But what if you don't have a nice GUI based tool? Or you want to work out for yourself what is really going on in Oracle? Or something has happened to the nice GUI tool and you cannot use it for one reason or another? Then you need to know about Dynamic Performance Views in Oracle, also known as the V$ tables, and which ones are the most relevant ones. The Dynamic Performance Views are views defined internal to Oracle that in fact map onto memory structures inside the SGA (System Global Area). They enable you to see in real time various data within the SGA using normal SQL queries.

When investigating performance problems you should generally narrow it down to a specific problem and the users being affected by it. In Oracle each connected user has a corresponding "session" inside the Oracle database instance, and information on each session is made available in the V$SESSION dynamic performance view.

This is both an advantage and disadvantage of the Dynamic Performance Views - they are a snapshot of current values in the SGA, and these values can be changing all of the time. Look again and you will get different results. Some views show cumulative values rather than current values, but they are always increasing over time and also constantly changing.

But with that in mind, you can use the V$SESSION view to look at what specific sessions are doing at the moment, and look for any slow, long running SQL statements. We can group the data columns into the following related groups:
  • Session Connection Information - User name, Client Application software, Logon time
  • Current Activity Information - Status, SQL statement executing
  • Delays - both normal Waits and abnormal Blocks i.e. small delays or completely blocked
    • Waits are delays that will definitely finish e.g. read a block from disk
    • Blocks are delays that are outside the control of the session and can potentially go on forever
      • e.g. blocked waiting for a row lock held by another session
For a full list of the columns in V$SESSION look in the "Oracle Database Reference" Manual under "Dynamic Performance Views", where you will get a brief description of each column.

All Active Sessions

Want a list of all active sessions and whether they are waiting or not? Try this query from SQL*Plus:
-- List the active sessions and break down key data
column sid              format 999999
column db_user          format a8      heading 'USER'
column db_login_time    format a6      heading 'LOGON'
column program          format a10     heading 'APP_N'
column active_for       format     999 heading 'ACTV|FOR'
column state            format a5      heading 'STATE'
column waiting_for      format     999 heading 'WAIT|FOR'
column event            format a12     heading 'WAIT ON'
column sql_id           format a13     heading 'SQL ID'
select s.sid, 
       nvl (s.username, 'SYS') db_user, 
       to_char (s.logon_time, 'hh24:mi') db_login_time,
       substr (s.program, 1, 10) program,
       s.last_call_et active_for,
       decode (s.state, 
                'WAITING', 'WAIT',
                'WAITED KNOWN TIME', 'NW L',
                'NW S') state,
       decode (s.wait_time, 0, seconds_in_wait, 0) waiting_for,
       substr (s.event, 1, 12) event, 
       nvl (s.sql_id, 'Not Executing') sql_id
  FROM v$session s
 WHERE s.type = 'USER'
   AND s.status = 'ACTIVE'
   AND s.wait_class != 'Idle'
Note the following about this query:
  • It restricts the sessions to only those from real users (not internal background sessions) that are active and experiencing non-idle waits.
    • Idle waits are generally waits external to Oracle, such as waiting for the next SQL statement to execute
  • The number of columns of data output is kept low to fit within an 80 column wide terminal screen
    • If you have a wider output format then you could add extra data columns to the "select"
  • The "decode" of "state" is meant to encode the following combinations of possibilities:
    • i.e. it combines both whether the session is currently waiting or not, with whether the previous wait was a short or long wait
  • It is assumed that users have all connected today, so only the time of connection is shown
    • If connections are long lived, then change the date format used in the "to_char" to include the day
From this you can see the following:
  • How many sessions are active, and how many are currently waiting on something?
    • Sessions that are active and not waiting are therefore running on a CPU
  • How many sessions are running the same SQL statement? Look at the values of SQL_ID
  • How many sessions are experiencing the same kind of wait event?
  • Are sessions experiencing long or short waits most of the time?

One Session's Details

If you know the SID (Session ID) of one session you are interested in and want to drill down further, then you could run the following set of queries from within SQL*Plus:
set newpage none
set define on
undefine SESSION_ID
accept SESSION_ID prompt 'Enter Session ID > '
column sid          FORMAT 99999
column username     FORMAT a12
column logt         FORMAT a12 HEADING 'Logon Time'
column sql_start    format a9  heading 'SQL Start'
column last_call_et            heading 'Time at Status'
column module       FORMAT a16 heading 'Module'
column client_info  format a16 heading 'Client Info'
column command      format 999 heading 'cmd'
column taddr        format a8  heading 'TX Addr'
column server                  heading 'Server|Type'
column schemaname   format a12 heading 'Schema|Name'
column type                    heading 'Session|Type'
column sql_id                  heading 'Curr SQL ID'
column prev_sql_id             heading 'Prev SQL ID'
column lockwait     format a8  heading 'Wait Lock|Addr'
column event        format a25 heading 'Waiting For'
column wait_class   format a12 heading 'Wait Class'
column row_wait_obj#           heading 'Object|Waiting On'
column wait_time               heading 'Last Wait Time|(0=Waiting)'
column seconds_in_wait         heading 'Elapsed From|Last Wait'
column blocking_session        heading 'Blocking|Session ID'
column blocking_session_status heading 'Blocking|Sess Status'
select sid, 
       to_char (logon_time, 'dd/mm hh24:mi') logt, 
       to_char (sql_exec_start, 'HH24:MI:SS') sql_start
from v$session
where sid = &&SESSION_ID
select sid, 
       substr (module, 1, 24) module, 
       substr (client_info, 1, 30) client_info,
       server, schemaname, type
from v$session
where sid = &&SESSION_ID
select sid, 
       sql_id, prev_sql_id, 
       substr (wait_class, 1, 20) wait_class
from v$session
where sid = &&SESSION_ID
select sid, 
       wait_time, seconds_in_wait,
       blocking_session, blocking_session_status
from v$session
where sid = &&SESSION_ID
column sql_text format a62
select sql_id, sql_text from v$sql
where sql_id = (select sql_id from v$session where sid = &&SESSION_ID)
undefine SESSION_ID
set newpage 1

This gives you a lot more information from V$SESSION for the one session, broken down into related sets of data, as described before. Note that the final query selects from V$SQL which is probably the other main Dynamic Performance View, which has an entry for each SQL statement currently in the library cache in the SGA. Remember that a SQL statement in the library cache can be shared between many sessions.

One Session Detailed Drill Down

If you really want to drill down more into what a single session or a set of sessions is doing over a period of time then you could use Tanel Poder's snapper.sql. This snapshots the contents of V$SESSION and other Dynamic Performance views over a period of time and reports on what it captured. Mainly this will be the wait events experienced by the sessions, but a lot of other activity statistics are reported too.

Snapper is written in PL/SQL and uses two in-memory PL/SQL tables for each snapshot, and then lists out anything that has changed between them. It does not write anything to the database, and only needs minimal permissions - SELECT_CATALOG_ROLE and execute on DBMS_LOCK.SLEEP. Thus it is safe to run on any database.


I've shown how to list all the currently active sessions on a database including the SQL they are executing and what they are waiting on, and how to get more details on what just one session is doing.

If you want even more information on what a single session is doing then you can SQL Trace it, which creates a trace file on the database server of all the SQL statements executed by that session and all the waits it experienced while tracing was enabled. Nothing is missed out because every executed SQL statement from that session is traced. You can then use utilities such as "tkprof" to post process the trace file and report on the SQL statements executed, their elapsed times, and the waits they experienced.

If the slow down is not currently happening but was very recent then you can look back at the recent past using ASH (Active Session History), but note that ASH is an extra cost option - you must have the Diagnostic Pack of the Enterprise Edition of Oracle. I'll write something about using ASH in a future post.

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