Tracing sessions

posted Jul 19, 2011, 12:22 PM by Sachchida Ojha
For Query Performance issues the typical requirement is to record wait and bind variable information for queries. This is achieved using 10046 with level 12.
The following examples outline how to set the event in various scenarios:
Trace Location
Session Tracing
Tracing a process after it has started
Instance wide tracing
Initialisation parameter setting
Tracing sessions via a logon trigger
Collecting Trace with SQLT
Tracing with DBMS_MONITOR

Trace Location

11g R1 and above:

With the introduction of the new diagnosability infrastructure introduced in Oracle Database 11g Release 1, traces and core files are placed in a location controlled by the DIAGNOSTIC_DEST initialization parameter.
To show the location of the DIAGNOSTIC_DEST, the following command can be used:
SQL> show parameter diagnostic_dest

Pre 11g R1:

Event 10046 tracing will produce a trace file in the <> for user processes and <> for background processes.
To show the location of the user_dump_dest, the following command can be used:
SQL> show parameter user_dump_dest

Note: Some examples include setting a 'tracefile_identifier' to assist with finding the resultant trace output.


Session Tracing

This tracing can be used where the session is accessible to the user prior to the start of the statement(s) to be traced.
To gather 10046 trace at the session level:

alter session set tracefile_identifier='10046';

alter session set timed_statistics = true;
alter session set statistics_level=all;
alter session set max_dump_file_size = unlimited;

alter session set events '10046 trace name context forever,level 12';

-- Execute the queries or operations to be traced here --

select * from dual;
exit;
If the session is not exited then the trace can be disabled using:

alter session set events '10046 trace name context off';
Note that if the session is not closed cleanly and tracing is disabled, then important trace information may be missing from the trace file.


Tracing a process after it has started

If trace from an existing session is required then oradebug can be used to attach to the session and initiate 10046 tracing.


The first step is to identify the session to be traced by some means:

For example, in SQL*Plus, start a session to find the OS process id (spid) for the target session:

select p.PID,p.SPID,s.SID
from v$process p,v$session s
where s.paddr = p.addr
and s.sid = &SESSION_ID
/
SPID is the operating system Process identifier (os pid)
PID is the Oracle Process identifier (ora pid)

Once the OS process id for the process has been determined then the trace can be initialised as follows:

Lets assume that the process to be traced has an os pid of 9834.
Login to SQL*Plus as a dba and execute the following:

connect / as sysdba
oradebug setospid 9834
oradebug unlimit
oradebug event 10046 trace name context forever,level 12
Remember to replace the example '9834' value with the actual os pid.


Note that it is also possible to attach to a session via oradebug using the 'setorapid'.
In this case the PID (Oracle Process identifier ) would be used (rather than the 'SPID') and the oradebug text would change to:
connect / as sysdba
oradebug setorapid 9834
oradebug unlimit
oradebug event 10046 trace name context forever,level 12
Remember to replace the example '9834' value with the actual ora pid.

To disable oradebug tracing once tracing is finished:
oradebug event 10046 trace name context off

Instance wide tracing


Note: Please be cautious when setting system wide, as this will impact performance due to every session being traced.

This setting will trace every session that is created after the parameter is set. Existing sessions will not be traced.

Setting system-wide 10046 tracing can be useful for scenarios where a problem session is known to occur but cannot be identified in advance.
In this situation, tracing can be enabled for a short period of time, the problem can then be reproduced and tracing disabled and the resultant traces searched for evidence of the problem.

System-wide tracing can be enabled as follows:

alter system set events '10046 trace name context forever,level 12';
The setting can be disabled in all sessions by using the following command:

alter system set events '10046 trace name context off';

Initialisation parameter setting

This setting will trace every session in the instance when it is restarted.

event="10046 trace name context forever,level 12"
The setting can be disabled by removing the parameter and restarting the instance or by using an alter system command as follows:

alter system set events '10046 trace name context off';

Via a Logon Trigger

There may be some situations where it is necessary to trace the activity of a specific user. In this case a logon trigger could be used.
An example is provided below:
CREATE OR REPLACE TRIGGER SYS.set_trace
AFTER LOGON ON DATABASE
WHEN (USER like '&USERNAME')
DECLARE
lcommand varchar(200);
BEGIN
EXECUTE IMMEDIATE 'alter session set statistics_level=ALL';
EXECUTE IMMEDIATE 'alter session set max_dump_file_size=UNLIMITED';
EXECUTE IMMEDIATE 'alter session set events ''10046 trace name context forever, level 12''';
END set_trace;
/


Note that in order to trace a session, the user executing the trigger
needs to have been explicitly granted 'alter session' privileges. i.e.:

grant alter session to <USERNAME> ;




Comments