Different user, different indexes used. Why?

When I connect using SQL*Plus to Oracle 8i DB using cost based optimization and run _identical_ queries (syntax and literals) as two different users, as one of the users the query takes less than a second and as the other user it takes over a minute. I did an explain plan on both and found that for the fast user unique indexes are used on all three tables in the query and for the slow user only one unique index is used while the other two indexes are ignored.

Can someone explain how this could happen?

Thanks,
Sam Received on Wed Feb 14 2007 - 12:45:26 CST