Serveroutput in the command window of PLSQL Developer

Question

In sqlplus, I input the following SQL:
SQL> set serveroutput on
SQL> show serveroutput
serveroutput ON SIZE UNLIMITED FORMAT WORD_WRAPPED
SQL> select /*+ no_index(t1 idx_t1) */ * from t1 where n=3;
     N
     3
SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
While I input the same SQL in PLSQL Developer:
SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
SQL_ID     9babjv8yq8ru3, child number 0
BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES); END;
NOTE: cannot fetch plan for SQL_ID: 9babjv8yq8ru3, CHILD_NUMBER: 0
Please verify value of SQL_ID and CHILD_NUMBER;
It could also be that the plan is no longer in cursor cache (check v$sql_plan)
And the most important is:
SQL> show serveroutput
SQL>
The result of it is empty.
The reason for that is serveroutput is only supported in sqlplus not PLSQL? I know serveroutput is sqlplus language not SQL. However I use "show serveroutput" in command window of PLSQL Developer which is the same as sqlplus. What's the real reason for that?
Thanks for your kindly reply.

Answer 1

" If serveroutput is on when you call this function, the last statement you will have run will be the (hidden) call to dbms_output that follows your execution of any other statement – so you won’t get the plan and statistics."
http://jonathanlewis.wordpress.com/2006/11/09/dbms_xplan-in-10g/
"Luckily I recognized the line BEGIN DBMS_OUTPUT.GET_LINES(...); END; from the tkprof output files. It is what SQL*Plus does after each statement: flushing the dbms_output buffer to the screen. To have the dbms_xplan.display_cursor show what I want, I started writing a little script against V$SQL to get the latest "real" SQL-statement, but then I realised I cannot possibly be the first to encounter this problem. Googling led me to this very useful blog entry by Jonathan Lewis. It contains almost everything you want to know about the display_cursor function. A simple "set serveroutput off" is enough to get the function working! "
http://rwijk.blogspot.nl/2008/03/dbmsxplandisplaycursor.html
So, actually you should have set serverout off.
Probably also depending on how you're gathering the execution plan statistics (you're not using the gather_plan_statistics hint?) and the SQL*Plus version and database version you're working with...