SERVEROUTPUT shows results one after another execution - oracle

Question

When I execute the following trigger code it is compiled. Then after inserting data, serveroutput says '1 row inserted' trigger result ('User Example has insrted a data') didn't displayed.
Then after execution of another PL/SQL the previous trigger result will appears on display. What should i do to solve this issue?
Here is the trigger and the insertion command and also the results,
/
SET SERVEROUTPUT ON;
CREATE OR REPLACE TRIGGER BTR_SUPERHEROS
BEFORE INSERT ON SUPERHEROS
FOR EACH ROW
ENABLE
DECLARE
V_USER VARCHAR2(30);
BEGIN
SELECT USER INTO V_USER FROM DUAL;
DBMS_OUTPUT.ENABLE();
DBMS_OUTPUT.PUT_LINE('USER:' ||V_USER||' HAS INSERTED A DATA');
END;
/
RESULT - Trigger BTR_SUPERHEROS compiled
INSERT INTO SUPERHEROS VALUES('SUPERMAN');
RESULT - 1 row inserted. (Didn't Shows the DBMS_OUTPUT)

Answer 1

You need to set serveroutput on in the client application (SQL*Plus, SQL Developer etc) in the session where you run a statement, otherwise it won't know that you want it fetch and display the output buffer when the statement completes. The set server output on prior to creating the trigger is just a client setting and doesn't set it permanently for the trigger.
Please confirm whether your test case was run from a single session, like this:
SQL*Plus: Release 12.1.0.1.0 Production on Tue Aug 15 08:17:48 2017
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Last Successful login time: Tue Aug 15 2017 08:14:08 +01:00
Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options
SQL> SET SERVEROUTPUT ON;
SQL>
SQL> CREATE OR REPLACE TRIGGER BTR_SUPERHEROS
2 BEFORE INSERT ON SUPERHEROS
3 FOR EACH ROW
4 ENABLE
5
6 DECLARE
7 V_USER VARCHAR2(30);
8 BEGIN
9 SELECT USER INTO V_USER FROM DUAL;
10 DBMS_OUTPUT.ENABLE();
11 DBMS_OUTPUT.PUT_LINE('USER:' ||V_USER||' HAS INSERTED A DATA');
12 END;
13 /
Trigger created.
SQL> INSERT INTO SUPERHEROS VALUES('SUPERMAN');
USER:WILLIAM HAS INSERTED A DATA
1 row created.
Note also that it is not recommended to call dbms_output.enable. From the documentation:
You should generally avoid having application code invoke either the DISABLE Procedure or ENABLE Procedure because this could subvert the attempt of an external tool like SQL*Plus to control whether or not to display output.
You can also assign values using the assignment := operator instead of a select into statement. The trigger could therefore be written as:
create or replace trigger btr_superheros
before insert on superheros
for each row
enable
declare
v_user user_users.username%type := sys_context('userenv', 'current_schema');
begin
dbms_output.put_line('User: ' ||v_user||' inserted value '''||:new.name||'''');
end;
/