Logon Triggers: The Oracle Database Magic Bullet

Posted on by lukaseder
Imagine you want to collect detailed usage statistics to tune your Oracle database, e.g. if you want to have A-Rows and A-Time values in your execution plans (by default, Oracle only reports E-Rows and E-Time with “E” for “Estimated”. But usually, you will care more about the “A” for “Actual”). All you have to do is connect with a sysdba user and issue the following statement: 
C:\> sqlplus "/ as sysdba"

Connected to:
Oracle Database 11g Express Edition Release 11.2.0.2.0

SQL> alter system set statistics_level = all;

System altered.
But of course, you hardly ever have the required privileges to connect as sysdba or to issue ALTER SYSTEM statements. So how can we get those advanced statistics? It’s easy, by using a logon trigger: 

CREATE OR REPLACE TRIGGER logon_actions
AFTER LOGON
ON DATABASE
ENABLE
BEGIN
    EXECUTE IMMEDIATE 
    'ALTER SESSION SET STATISTICS_LEVEL = all';
END;
/
This will set your session’s statistics level to all every time you log on to the database. Of course, you will need to have the privilege to create such a system trigger, but maybe your DBA will allow you to specify a certain package that provides a debug level for things like these: 

DECLARE
    v_loglevel VARCHAR2(100);
BEGIN
    v_loglevel := logger_package.loglevel;

    IF v_loglevel = 'DEBUG' THEN
        EXECUTE IMMEDIATE 
        'ALTER SESSION SET STATISTICS_LEVEL = all';
    END IF;
END;
To be sure that the statistics_level has been set correctly, run this query: 

SELECT SID, NAME, VALUE
FROM   V$SES_OPTIMIZER_ENV
WHERE  NAME = 'statistics_level'
AND SID = (
    SELECT SID
    FROM V$MYSTAT
    WHERE ROWNUM = 1
);
To learn about how to get A-Rows and A-Time values in your execution plan, read this article here. Happy statistics collecting!

Published by lukaseder

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