A question about the impact of SQL*PLUS SERVEROUTPUT option on v$sql

Question

Hello everybody,
SQL> SELECT * FROM v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE    11.2.0.1.0  Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
SQL>
OS : Fedora Core 17 (X86_64) Kernel 3.6.6-1.fc17.x86_64I would like to ask a question about the SQL*Plus SET SERVEROUTPUT ON/OFF option and its impact on queries on views such as v$sql and v$session. Here is the problem
Actually I define three variables in SQL*Plus in order to store sid, serial# and prev_sql_id columns from v$session in order to be able to use them later, several times in different other queries, while I'm still working in the current session.
So, here is how I proceed
SET SERVEROUTPUT ON;  -- I often activate this option as the first line of almost all of my SQL-PL/SQL script files
SET SQLBLANKLINES ON;
VARIABLE mysid NUMBER
VARIABLE myserial# NUMBER;
VARIABLE saved_sql_id VARCHAR2(13);
-- So first I store sid and serial# for the current session
BEGIN
    SELECT sid, serial# INTO :mysid, :myserial#
    FROM v$session
    WHERE audsid = SYS_CONTEXT('UserEnv', 'SessionId');
END;
PL/SQL procedure successfully completed.
-- Just check to see the result
SQL> SELECT :mysid, :myserial# FROM DUAL;
    :MYSID :MYSERIAL#
       129   1067
SQL> Now, let's say that I want to run the following query as the last SQL statement run within my current session
SELECT * FROM employees WHERE salary >= 2800 AND ROWNUM <= 10;According to Oracle® Database Reference 11g Release 2 (11.2) description for v$session
http://docs.oracle.com/cd/E11882_01/server.112/e25513/dynviews_3016.htm#REFRN30223]
the column prev_sql_id includes the sql_id of the last sql statement executed for the given sid and serial# which in the case of my example, it will be the above mentioned SELECT query on the employees table. As a result, right after the SELECT statement on the employees table I run the following
BEGIN
    SELECT prev_sql_id INTO :saved_sql_id
    FROM v$session
    WHERE sid = :mysid AND serial# = :myserial#;
END;
PL/SQL procedure successfully completed.
SQL> SELECT :saved_sql_id FROM DUAL;
:SAVED_SQL_ID
9babjv8yq8ru3
SQL> Having the value of sql_id, I'm supposed to find all information about cursor(s) for my SELECT statement and also its sql_text value in v$sql. Yet here is what I get when I query v$sql upon the stored sql_id
SELECT child_number, sql_id, sql_text
FROM v$sql
WHERE sql_id = :saved_sql_id;
CHILD_NUMBER   SQL_ID          SQL_TEXT
0              9babjv8yq8ru3    BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES); END;Therefore instead of
SELECT * FROM employees WHERE salary >= 2800 AND ROWNUM <= 10;for the value of sql_text I get the following value
BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES);Which is not of course what I was expecting to find in v$sql for the given sql_id.
After a bit googling I found the following thread on the OTN forum where it had been suggested (well I think maybe not exactly for the same problem) to turn off SERVEROUTPUT.
Problem with dbms_xplan.display_cursor
This was precisely what I did
SET SERVEROUTPUT OFFafter that I repeated the whole procedure and this time everything worked pretty well as expected. I checked SQL*Plus documentation for SERVEROUTPUT
and also v$session page, yet I didn't find anything indicating that SERVEROUTPUT should be switched off whenever views such as v$sql, v$session
are queired. I don't really understand the link in terms of impact that one can have on the other or better to say rather, why there is an impact
Could anyone kindly make some clarification?
thanks in advance,
Regards,
Dariyoosh

Answer 1

>
and also v$session page, yet I didn't find anything indicating that SERVEROUTPUT should be switched off whenever views such as v$sql, v$session
are queired. I don't really understand the link in terms of impact that one can have on the other or better to say rather, why there is an impact
Hi Dariyoosh,
SET SERVEROUTPUT ON has the effect of executing dbms_output.get_lines after each and every statement. Not only related to system view.
Here below what Tom Kyte is explaining in this page:
Now, sqlplus sees this functionality and says "hey, would not it be nice for me to dump this buffer to screen for the user?". So, they added the SQLPlus command "set serveroutput on" which does two things
1) it tells SQLPLUS you would like it <b>to execute dbms_output.get_lines after each and every statement</b>. You would like it to do this network rounding after each call. You would like this extra overhead to take place (think of an install script with hundreds/thousands of statements to be executed -- perhaps, just perhaps you don't want this extra call after every call)
2) SQLPLUS automatically calls the dbms_output API "enable" to turn on the buffering that happens in the package.Regards.
Al