By Laurent Schneider
There is a new command in 11g to display the current events.
It is oradebug eventdump.
For example :
SQL> alter session set events '10046 trace name context
forever,level 12:942 trace name ERRORSTACK level 3';
SQL>
oradebug setmypid
Statement processed.
SQL> oradebug
eventdump session
sql_trace level=12 942
trace name
ERRORSTACK level 3
Read metalink note 436036.1
In 10g and before, the command was oradebug dump events 1 and the
list was dumped in a trace file. 11g directly outputs to the
console.
Note: There is no backward compatibility with unsupported
tools like oradebug.
In 11g, you will get an ORA-00076 with dump events:
SQL> oradebug setmypid
Statement processed.
SQL> oradebug dump events 1
ORA-00076: dump EVENTS not found
$ oerr ora 76
00076, 00000, "dump %s not found"
//
*Cause: An attempt was made to invoke a dump that does not exist.
// *Action: Type DUMPLIST to see the list of available dumps.
Or, how many rows would be returned by:
select * from dual where dbms_random.value=dbms_random.value;
It is wrong to assume the function will be evaluated twice.
The short answer would be : do not rely on
random plsql functions in SQL�
Here is a test case in 11.2.0.2 and 10.2.0.3
SQL> select version from v$instance;
VERSION
-----------------
10.2.0.3.0
SQL> select *
from dual where dbms_random.value=dbms_random.value;
no rows
selected
Execution Plan
----------------------------------------------------------
Plan hash value: 1224005312
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 2 | 2 (0)|
00:00:01 |
|* 1 | FILTER | | |
| | |
| 2 | TABLE ACCESS FULL| DUAL
| 1 | 2 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 -
filter("DBMS_RANDOM"."VALUE"()="DBMS_RANDOM"."VALUE"())
In 10g, the function is executed twice per row, and the chance to
have two different values is more than 99.9999�%.
SQL> select version from v$instance;
VERSION
-----------------
11.2.0.2.0
SQL> select *
from dual where dbms_random.value=dbms_random.value D - X
Execution Plan
----------------------------------------------------------
Plan hash value: 1224005312
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
Time
|--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 2 | 2 (0)|
00:00:01 |
|* 1 | FILTER | | |
| | |
| 2 | TABLE ACCESS FULL| DUAL
| 1 | 2 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 -
filter("DBMS_RANDOM"."VALUE"() IS NOT NULL)
Here the optimized execute the function only once per row,
and since the result is never null, it always evaluates to true.
Is this a bug or a feature?
In my opinion it is a confusing tuning enhancement that may break
badsome programs.
In the discussion, I mentioned that prior dbms_random.value IS
NOT NULL is an unsafe construct.
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