10g R2 NVARCHAR2 check constraint problems

Question

Hi, Thanks for taking a look...
I've just installed 10g R2 on Solaris 10 and run into a small problem with NVARCHAR2s.
I have a table with a check constraint to make sure the column is a valid value on an nvarchar2 column. Querying the table for a specific value results in no data found, however the data appears to be intact.
The database NLS_CHARACTERSET is AL32UTF8 and NLS_NCHAR_CHARACTERSET is AL16UTF16.
I think the following script results can show the problem better than I can explain it.
Any suggestions would be appreciated. Thanks.
Mike
SQL*Plus: Release 10.2.0.1.0 - Production on Mon Apr 17 13:51:40 2006
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bit Production
With the Partitioning and Data Mining options
SQL> @problem_script.sql
SQL> create table foo (
2 bar nvarchar2(10) constraint ck_bar check (bar in ('abc','def'))
3 );
Table created.
SQL> insert into foo values ('abc');
1 row created.
SQL> commit;
Commit complete.
SQL>
SQL> REM Strange. This worked in every version prior to 10g R2.
SQL> REM Why doesn't it work now?
SQL>
SQL> select * from foo where bar = 'abc';
no rows selected
SQL>
SQL> REM Even more strange is why this doesn't work:
SQL>
SQL> select * from foo where bar = N'abc';
no rows selected
SQL>
SQL> REM Stranger still are these two queries:
SQL>
SQL> select bar, decode(bar, 'abc', 'It matches', 'No it doesnt')
2 from foo;
BAR DECODE(BAR,'A
abc It matches
SQL>
SQL> REM This is still strange, but not unexpected given the prior query
SQL>
SQL> select bar, decode(bar, 'abc', 'It matches', 'No it doesnt')
2 from foo
3 where bar = 'abc';
no rows selected
SQL>
SQL> REM Strangest of all is why this DOES work:
SQL>
SQL> alter table foo disable constraint ck_bar;
Table altered.
SQL> select * from foo where bar = 'abc';
BAR
abc
SQL>
SQL>
SQL> REM Regular varchars are unchanged...
SQL>
SQL> create table bar (
2 foo varchar2(10) constraint ck_foo check (foo in ('abc','def'))
3 );
Table created.
SQL> insert into bar values ('abc');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from bar where foo = 'abc';
FOO
abc

Answer 1

Nope.
SQL> select * from foo where bar = TO_NCHAR('abc');
no rows selected
Of all the things I've tried (and I think I've tried just about everything), only the TO_CHAR(bar) and disabling the check constraint work. The fact that DECODE(bar, 'abc', 'true', 'false') in the select list is also confusing since it's converted there correctly, but not in the WHERE clause. And why does it work as epxected when the constraint is disabled?
My thoughts are that it can't be an Oracle bug, otherwise I'm sure I wouldn't be the first to find it. I've built the instance using the same configuration script I used for the 10g R1 database that works, so the configuration parametes are the same.
More things that don't make sense:
SQL> select dump(bar), dump('abc') from foo;
DUMP(BAR)
DUMP('ABC')
Typ=1 Len=6: 0,97,0,98,0,99
Typ=96 Len=3: 97,98,99
SQL> select dump(bar), dump(to_nchar('abc')) from foo;
DUMP(BAR)
DUMP(TO_NCHAR('ABC'))
Typ=1 Len=6: 0,97,0,98,0,99
Typ=1 Len=6: 0,97,0,98,0,99
Thanks for your help though...
Mike