WHERE clauses and Merge Join Cartesian?

Question

For some reason, Siebel is generating queries like this:
                    AND CONCAT (CONCAT (t41828.lvl8anc_postn, '-'),
                                t41828.lvl8_emp_full_name
                               ) = 'GEO-SMITH, BILL'
                    AND CONCAT (CONCAT (t41828.lvl6anc_postn, '-'),
                                t41828.lvl6_emp_full_name
                               ) = 'GROUP1-DOE, JOHN'and this ends up with 3 merge join cartesian in autotrace and this query takes several hours.
However, by rewriting just those 2 clauses to:
                    AND (t41828.lvl8anc_postn = 'GEO' and
                                t41828.lvl8_emp_full_name = 'SMITH, BILL')
                    AND (t41828.lvl6anc_postn = 'GROUP1' and
                                t41828.lvl6_emp_full_name = 'DOE, JOHN')the merge join cartesians go away, and it runs in 7 seconds.
However, since Siebel is generating the query, and we are having issues, we decided to add columns for testing that is equivalent to the concatenation of the two columns above. After doing this, and creating indexes on the new columns, it is now back to the merge join cartesian and I cannot get rid of it.
So:
1. How can rewriting the WHERE (not adding or deleting anything here, just rewriting it in a different way) clause eliminate the merge join cartesian?
I'm guessing by understanding that single question, I will be able to come up with a better solution to this.

Answer 1

Note we made two additional columns:
CONCAT (CONCAT (t41828.lvl8anc_postn, '-'), t41828.lvl8_emp_full_name) => col1 CONCAT (CONCAT (t41828.lvl6anc_postn, '-'), t41828.lvl6_emp_full_name) => col2
When I created indexes on col1 and col2 and used them in the query, the cartesians returned, and I cannot figure out why. So, I'm still confused why:
In the original query:
1. Using CONCAT, and thus no indexes => cartesian
2. No CONCAT, and indexes => no cartesian
With the new columns, col1 and col2:
1. No CONCAT needed, full table scan or indexes => cartesian