Full table scan on query using primary key?

[gandolf989]

I am using Oracle 9.2.0.6 on Windows 2000. I have the following query which was generated from Java. When I try to get an explain plan it is telling me that it is doing a full table scan on uln, even though tpfdd_runid_uln is the primary key. If I hint the index, the cost goes way up but the response time is infinitely faster. Can anyone explain why this is? I enclosed the files, they are html files saved with a .txt extension so that I could upload them.

  SELECT uln_syn.poe_ald_cal AS grp0, SUM ( uln_syn.sumoftotal_stons ) tot0
    FROM uln_syn
   WHERE EXISTS (
            SELECT ca_keys.comp_key
              FROM ca_keys
             WHERE ca_keys.comp_id = '0x080e00000107235e9e7f0a1428098124'
					AND uln_syn.tpfdd_runid_uln = ca_keys.comp_key)
  GROUP BY uln_syn.poe_ald_cal;