Oracle query against SQL Server database giving inconsistent results

We have multiple Oracle 11.2.0.2 databases with og4odbc database links to a SQL Server 2008 database. A particular table in the SQL Server database has two Varchar(36) columns. The following statement returns 1292 rows:

select "id", "folder_id" from dbo.mediasite_presentation@ms;

Querying for a specific value in the first column returns a row:

select "id", "folder_id" from dbo.mediasite_presentation@ms 
   where "id"='006d815d-6e9e-4004-9104-51213a1ecd52';
id                                   folder_id
------------------------------------ ------------------------------------
006d815d-6e9e-4004-9104-51213a1ecd52 54aa9b6e-1c55-4de5-a06f-033d8b19fff0

But querying for a specific value in the second column returns no rows:

select "id", "folder_id" from dbo.mediasite_presentation@ms 
   where "folder_id"='54aa9b6e-1c55-4de5-a06f-033d8b19fff0';
no rows selected

Using the where clause that works and wrapping each field in a to_char returns the following interesting results:

select to_char("id"), to_char("folder_id") from dbo.mediasite_presentation@ms 
   where "id"='006d815d-6e9e-4004-9104-51213a1ecd52';
TO_CHAR("ID")                        TO_CHAR("FOLDER_ID")
------------------------------------ ------------------------------------
006d815d-6e9e-4004-9104-51213a1ecd52 ┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐

Interestingly enough I can produce the same results just by adding a rownum clause:

select "id", "folder_id" from dbo.mediasite_presentation@ms 
   where "id"='006d815d-6e9e-4004-9104-51213a1ecd52' and rownum<=1;

id                                   folder_id
------------------------------------ ------------------
006d815d-6e9e-4004-9104-51213a1ecd52 ┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐

Even stranger is the fact that creating a materialized view for query that returns no data actually returns data.

```
Create Materialized View TempMV as (
select "id", "folder_id" from dbo.mediasite_presentation@ms
where "folder_id"='54aa9b6e-1c55-4de5-a06f-033d

I'd suspect some for of characterset conversion issue.
When you do a TO_CHAR, you are getting back "┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐┐" (18 characters). That suggests that there is some confusion about whether the data contained in the column is single-byte or multi-byte.

"The uniqueidentifier data type in SQL Server is stored natively as a 16-byte binary value"
This should ideally come across to Oracle as a RAW datatype not a VARCHAR2 or CHAR.

If you do a

CREATE VIEW temp_view as select  "id", "folder_id" from dbo.mediasite_presentation@ms

what shows up when you do a DESC temp_view

StackExchange Database Administrators Q#1825, answer score: 4