Populating OUT parameters when TOO_MANY_ROWS exception is thrown

Question

When I was trying to write code to test out today's PL/SQL challenge quiz, the behavior appears to depend on the table being queried. I can't figure out what attribute of the table drives the behavior (or if there is a different explanation for the behavior).
The quiz is testing what happens when a procedure does a SELECT INTO an OUT parameter and a TOO_MANY_ROWS exception is thrown. The intent is to show that even though the behavior is technically undefined, what actually happens is that the OUT parameter is populated with the first row that is selected (obviously, you would never write code that depends on this behavior-- this is solely an academic exercise). The demonstration code works as expected
CREATE TABLE plch_emp ( emp_name VARCHAR2(100) );
INSERT INTO plch_emp VALUES ('Jones');
INSERT INTO plch_emp VALUES ('Smith');
COMMIT;
CREATE OR REPLACE PROCEDURE plch_get
  (out_name OUT plch_emp.emp_name%TYPE) IS
BEGIN
  SELECT emp_name
  INTO out_name
  FROM plch_emp
  ORDER BY emp_name;
EXCEPTION
  WHEN OTHERS THEN
    dbms_output.put_line('A:' || out_name);
END;
What will be displayed after executing the following block:
DECLARE
  l_name plch_emp.emp_name%TYPE;
BEGIN
  plch_get(l_name);
  dbms_output.put_line('B:' || l_name);
END;
/and outputs
A:Jones
B:JonesWhen I replicate the logic while hitting the EMP table, the PLCH_EMP table, and the newly created EMP2 table, however, I get different behavior for the EMP and EMP2 tables. The procedure that queries PLCH_EMP works as expected but the OUT parameter is NULL when either EMP or EMP2 are created. Any idea what causes the behavior to differ?
select *
  from v$version;
create table emp2
as
select *
  from emp;
create or replace procedure p1( p_out out varchar2 )
is
begin
  select emp_name
    into p_out
    from plch_emp
   order by emp_name;
exception
  when others then
    dbms_output.put_line( 'P1 A:' || p_out );
end;
create or replace procedure p2( p_out out varchar2 )
is
begin
  select ename
    into p_out
    from emp
   order by ename;
exception
  when others then
    dbms_output.put_line( 'P2 A:' || p_out );
end;
create or replace procedure p3( p_out out varchar2 )
is
begin
  select ename
    into p_out
    from emp2
   order by ename;
exception
  when others then
    dbms_output.put_line( 'P3 A:' || p_out );
end;
declare
  l_ename varchar2(100);
begin
  p1( l_ename );
  dbms_output.put_line( 'P1 B:' || l_ename );
  p2( l_ename );
  dbms_output.put_line( 'P2 B:' || l_ename );
  p3( l_ename );
  dbms_output.put_line( 'P3 B:' || l_ename );
end;
SQL> select *
  2    from v$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 64-bit Windows: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
SQL>
SQL> create table emp2
  2  as
  3  select *
  4    from emp;
Table created.
SQL>
SQL> create or replace procedure p1( p_out out varchar2 )
  2  is
  3  begin
  4    select emp_name
  5      into p_out
  6      from plch_emp
  7     order by emp_name;
  8  exception
  9    when others then
10      dbms_output.put_line( 'P1 A:' || p_out );
11  end;
12  /
Procedure created.
SQL>
SQL> create or replace procedure p2( p_out out varchar2 )
  2  is
  3  begin
  4    select ename
  5      into p_out
  6      from emp
  7     order by ename;
  8  exception
  9    when others then
10      dbms_output.put_line( 'P2 A:' || p_out );
11  end;
12  /
Procedure created.
SQL>
SQL> create or replace procedure p3( p_out out varchar2 )
  2  is
  3  begin
  4    select ename
  5      into p_out
  6      from emp2
  7     order by ename;
  8  exception
  9    when others then
10      dbms_output.put_line( 'P3 A:' || p_out );
11  end;
12  /
Procedure created.
SQL>
SQL> declare
  2    l_ename varchar2(100);
  3  begin
  4    p1( l_ename );
  5    dbms_output.put_line( 'P1 B:' || l_ename );
  6
  7    p2( l_ename );
  8    dbms_output.put_line( 'P2 B:' || l_ename );
  9
10    p3( l_ename );
11    dbms_output.put_line( 'P3 B:' || l_ename );
12
13  end;
14  /
P1 A:Jones
P1 B:Jones
P2 A:
P2 B:
P3 A:
P3 B:
PL/SQL procedure successfully completed.Justin

Answer 1

Billy  Verreynne  wrote:
So we can then reasonably assume that the test or environment itself somehow interferes with the results? After all, the very same cursor is executed and the same PL/SQL engine uses that cursor interface.Clearly, there is something in my environment that is wonky. It just bugs me that I can't figure out what that variable is.
Your test was done as a single anonymous PL/SQL block. Which means each proc that was called referenced the same variable/memory - are there perhaps PL/SQL optimisation enabled for the database or session? Or any other settings that could influence the test? Have you tried calling the procedures directly from SQL*Plus using a bind var and not via an anon block and a local var in that block?I have. And the results were the same
SQL> var ename varchar2(100);
SQL> exec p1( :ename );
P1 A:Jones
PL/SQL procedure successfully completed.
SQL> exec p2( :ename );
P2 A:
PL/SQL procedure successfully completed.
SQL> exec p3( :ename );
P3 A:
PL/SQL procedure successfully completed.
As a sanity test - what happens when proc P3 for example is changed to a catersian join/union of emp2 and plch_emp? Do the test results change? Or you can change it as follows:
SQL> ed
Wrote file afiedt.buf
  1  create or replace procedure p5( p_out out varchar2 )
  2  is
  3  begin
  4    --// force a value into p_out using a successful fetch
  5    select emp_name
  6      into p_out
  7      from plch_emp
  8     where rownum = 1;
  9    --// force an error and determine if p_out was overwritten
10    select ename
11      into p_out
12      from emp2
13     order by ename;
14  exception
15    when others then
16      dbms_output.put_line( 'P5:' || p_out );
17* end;
18  /
Procedure created.
SQL> exec p5( :ename );
P5:Jones
PL/SQL procedure successfully completed.
Of course, you can also put this one down to using an operating system like Windows as a poor database server and not as a magnificent client gaming platform... Well, sure. But you can probably only play World of Warcraft so long before you need to write some PL/SQL.
Justin