SQL statement performance issues

Question

Hi: A sql statement I have is taking too long to complete. But if I break the statement into two separate statements, it runs fine.
SQL 1 runs fine and returns 163.
SELECT /*+ FIRST_ROWS */ entity_group.entity FROM auth, entity_group, user_group WHERE entity_group.auth_group = user_group.groupid and user_group.userid = auth.id and auth.username = 'ing';
SQL 2 runs fine, prints about 20 records
select /*+ FIRST_ROWS */ EVENT_DATA.info from EVENT_DATA, AGENTS where EVENT_DATA.agent_id = agents.id AND agents.entity IN (163);
SQL 3 takes a long time to run. The field AGENT_ID is indexed in EVENT_DATA.
select /*+ FIRST_ROWS */ EVENT_DATA.info from EVENT_DATA, AGENTS where EVENT_DATA.agent_id = agents.id AND agents.entity IN (SELECT /*+ FIRST_ROWS */ entity_group.entity FROM auth, entity_group, user_group WHERE entity_group.auth_group = user_group.groupid and user_group.userid = auth.id and auth.username = 'ing');
Any suggestions on improving the SQL statement or finding out if anything is wrong with the database.
Thanks
Ravi

Answer 1

First, lose the hints, analyse your tables and let the CBO do its job. If it is not fast enough. then Nicolas' solution may be faster.
Another possible construct would be:
SELECT event_data.info
FROM event_data, agents,
     (SELECT DISTINCT entity_group.entity
      FROM auth, entity_group, user_group
      WHERE entity_group.auth_group = user_group.groupid and
            user_group.userid = auth.id and
            auth.username = 'ing') eg
WHERE event_data.agent_id = agents.id and
      agents.entity = eg.entityTTFN
John