optimizing a (simple?) query on a largeish table

Hi.

I'm just getting started with PostgreSQL.  Porting over an huge Oracle
database application for a fun first project :^)

I've got the following query which I'm trying to run on a 4.2 million row table:

SELECT ActionItems.*
FROM ActionItems
WHERE
    attn=upper(SESSION_USER)
    or attn in (
        select upper(groname)
        from pg_group
        where (select oid from pg_roles where rolname = SESSION_USER) = ANY(grolist)
    )
ORDER BY dateTimeCreated

That is, "match any ActionItem directed to me personally, or to the
groups to which I belong".

It currently takes about 8 seconds.

I have indexes on both used columns in the large table ("attn" and
"dateTimeCreated"), but it doesn't seem to be using them --- I've
attached the "EXPLAIN" result below.

Any ideas about what's going on here?   How can I reduce the execution time?

Thanks,
Kurt

---

Sort  (cost=1242644.46..1247909.54 rows=2106033 width=200)
  Sort Key: datetimecreated
  ->  Seq Scan on actionitems  (cost=573.01..186430.80 rows=2106033 width=200)
        Filter: (((attn)::text = upper(("session_user"())::text)) OR
(hashed subplan))
        SubPlan
          ->  Seq Scan on pg_authid  (cost=5.10..573.01 rows=2 width=64)
                Filter: ((NOT rolcanlogin) AND ($0 = ANY ((subplan))))
                InitPlan
                  ->  Seq Scan on pg_authid  (cost=0.00..5.10 rows=1 width=4)
                        Filter: (rolname = "session_user"())
                SubPlan
                  ->  Seq Scan on pg_auth_members  (cost=0.00..4.01
rows=15 width=4)
                        Filter: (roleid = $1)