WARNING The misuse of events can cause damages and/or loss of service.
Contact Oracle Support to use events on Production databases.
Be carefulo a mistake on one digit or reversing two numbers can cause tragic consequences.
An event is a
· trace of a specific activity you can enable, to diagnose a problem
· dump information on error or at request
· task you can execute
· feature you can enable
· corruption you want to generate

There is few official information about events to use it, means a lot of investigation, but anyway the results deserves the effort, one example is the event 10053 to capture CBO information, you’ll find information about it digging on internet from people who had made investigation and discovered part of his functionality, or in books, for example you can get more information on the event 10046, from Cary’s book “Optimizing Oracle Performance” than in metalink.

Whenever you can use a supported package, DBMS_MONITOR to enable trace instead of event 10046, for example.

1.A Syntax

To enable an event you execute the command
ALTER [SESSION|SYSTEM] SET EVENTS =
‘(Event Number) TRACE NAME CONTEXT [FOREVER, LEVEL (Level Number) | OFF ]’;
EVENT=”(Event Number) TRACE NAME CONTEXT [FOREVER, LEVEL (Level Number) | OFF ]”

1.B Finding Specific Events

I had created a function table to search on errors messages, events are between 10000 and 10999 error message list.
Here is an example of how to use it.
SELECT CDESC FROM THE ( SELECT CAST(M.FUT_ERRORES(10000,10999) AS M.TYP_ERRORES ) FROM DUAL )
WHERE UPPER(CDESC) LIKE UPPER('%HISTOGRAM%') ; 1.B.i Error Trace ALTER [SESSION|SYSTEM] SET EVENTS =
‘(Error Number) TRACE NAME ERROR [STACK LEVEL (level)| OFF ]’;
In parameter file, you can set in the same way 1.B.i.a Hunting Events I’m getting an error and want more information, to debug faster, I want to know if there is a event to know which parameter is the one giving the error, so I enable trace. In this example I’m sending to the parameter c varchar2, a number.
create or replace procedure test_par(a varchar2,b varchar2,
c number, d varchar2,
e varchar2, f varchar2,
g varchar2, h varchar2,
i varchar, j varchar2) is
begin
null;
end;
/
ALTER SESSION SET EVENTS='6502 trace name ERRORSTACK level 1';
SQL> exec test_par('a','b','c','d','e','f','g','h','i','j');
BEGIN test_par('a','b','c','d','e','f','g','h','i','j'); END;
*
ERROR en lÝnea 1:
ORA-06502: PL/SQL: error : error de conversi¾n de carßcter a n·mero numÚrico o de valor
ORA-06512: en lÝnea 1
And I got this information, as you can see this don’t helps too much in finding the problem; unless you learn to interpret this. So we had to see another chances
I searched for a specific event
1 SELECT CDESC FROM THE ( SELECT CAST(M.FUT_ERRORES(10000,10999) AS M.TYP_ERRORES ) FROM DUAL )
2* WHERE UPPER(CDESC) LIKE UPPER('%PL%SQL%')
SQL> /
CDESC
-----------------------------------------------------------------------------------------------------
Event 10255: pl/sql parse checking
Event 10840: trace / debug pl/sql caching module (kkxmInitCache)

I searched , this took a time but on internet and found the event 10938
Then went to metalink and found
Note:52481.1 PL/SQL Tracing Features Available with PL/SQL 2.3.4 in Oracle 7.3.4,
I did several tests, but I didn’t found useful for my purpose.

So as you can see understand an event takes times, and not always you get what you expect, or you can’t understand all the data generated.