Problem with merge using package (row)variable

Question

We're doing an ETL project, where we have to validate data coming from staging tables before inserting / updating them in our data tables.
We use a validate function for this, in which we 'test' our mappings and business logic, and returns 'OK' if no errors were occured.This function is called in the WHERE clause, and populates a package row variable. From this variable, values are read in the insert / update statement, using a get function.
The problem is that sometimes, with a data set with NO duplicate keys, we get an ORA-00001 error, and sometimes not.
Does anyone have any idea why this error could occur ? We're on Oracle 9.2.0.5.0.
For the demo mentioned below, the following statement fails , while it succeeds if we add 'and rownum < 11' to the where clause :
merge into target_tab t
using (select st.* from source_tab st where demo_pack.assign_var(st.attrib_1, st.attrib_2, st.attrib_3) = 'OK') s
on (t.attrib_1 = s.attrib_1 and
t.attrib_2 = s.attrib_2)
when matched then update
set t.attrib_3 = demo_pack.get_attrib('ATTRIB_3')
when not matched then
insert(attrib_1, attrib_2, attrib_3)
values (demo_pack.get_attrib('ATTRIB_1'), demo_pack.get_attrib('ATTRIB_2'), demo_pack.get_attrib('ATTRIB_3'));
Can anyone explain to me why this statement sometimes fails, and other times not ?
Thanks in advance .
demo tables / packages :
create table source_tab
attrib_1 varchar2(30),
attrib_2 varchar2(30),
attrib_3 varchar2(200)
create table target_tab
attrib_1 varchar2(30) not null,
attrib_2 varchar2(30) not null,
attrib_3 varchar2(200),
constraint pk_target_tab primary key(attrib_1, attrib_2)
insert into source_tab
select table_name, column_name, data_type
from user_tab_columns;
commit;
create or replace package demo_pack as
function assign_var(p_attrib_1 in target_tab.attrib_1%type,
p_attrib_2 in target_tab.attrib_2%type,
p_attrib_3 in target_tab.attrib_3%type) return varchar2;
function get_attrib(p_name in varchar2) return varchar2;
end;
create or replace package body demo_pack as
g_rec target_tab%rowtype;
function assign_var(p_attrib_1 in target_tab.attrib_1%type,
p_attrib_2 in target_tab.attrib_2%type,
p_attrib_3 in target_tab.attrib_3%type) return varchar2 is
begin
g_rec.attrib_1 := p_attrib_1;
g_rec.attrib_2 := p_attrib_2;
g_rec.attrib_3 := p_attrib_3;
return 'OK';
end;
function get_attrib(p_name in varchar2) return varchar2 is
l_return varchar2(200);
begin
if p_name = 'ATTRIB_1' then
l_return := g_rec.attrib_1;
elsif p_name = 'ATTRIB_2' then
l_return := g_rec.attrib_2;
elsif p_name = 'ATTRIB_3' then
l_return := g_rec.attrib_3;
end if;
return l_return;
end;
end;
/

Answer 1

It's not necessarily that the problem is within your package code. As long as UNIQUE constraint exists on DEST table, any MERGE statement may fail with ORA-00001 due to concurrent updates and inserts taking place.
Of course, it's just my guess but consider the following scenario (three sessions modify DEST table in parallel – and to keep this example clear, I put their commands in chronological order):
S#1>  create table dest(x primary key, y) as
  2   select rownum, rownum
  3     from dual
  4  connect by level <= 5;
Table created.
S#1>  create table src(x, y) as
  2   select rownum + 1, rownum + 1
  3     from dual
  4  connect by level <= 5;
Table created.
S#1> select * from src;
         X          Y
         2          2
         3          3
         4          4
         5          5
         6          6
S#1> select * from dest;
         X          Y
         1          1
         2          2
         3          3
         4          4
         5          5
S#2> -- Now, session #2 will update one row in DEST table
S#2> update dest
  2     set y = 40
  3   where x = 4;
1 row updated.
S#2> select * from dest;
         X          Y
         1          1
         2          2
         3          3
         4         40
         5          5
S#1> -- Session #1 issues the following MERGE:
S#1> merge into dest d
  2  using (select * from src) s
  3     on (s.x = d.x)
  4   when matched then update set d.y = s.y
  5   when not matched then insert (d.x, d.y)
  6        values (s.x, s.y);
-- At this point, session #1 becomes blocked as it can not modify row locked by session #2
S#3> -- Now, session #3 inserts new row into DEST and commits.
S#3> -- MERGE in session #1 is still blocked.
S#3> insert into dest values (6, 6);
1 row created.
S#3> select * from dest;
         X          Y
         1          1
         2          2
         3          3
         4          4
         5          5
         6          6
6 rows selected.
S#3> commit;
Commit complete.
S#2> -- Session #2 rolls back its UPDATE.
S#2> rollback;
Rollback complete.
-- Finally, session #1 is getting unblocked, but...
merge into dest d
ERROR at line 1:
ORA-00001: unique constraint (MAX.SYS_C0032125) violated
S#1>Hope this helps,
Andrew.