Improve performance of query

I'm trying to port our application from MS-SQL to Postgres. We have
implemented all of our rather complicated application security in the
database. The query that follows takes a half of a second or less on
MS-SQL server and around 5 seconds on Postgres. My concern is that this
data set is rather "small" by our applications standards. It is not
unusual for the da_answer table to have 2-4 million records. I'm
worried that if this very small data set is taking 5 seconds, then a
"regular sized" data set will take far too long.

I originally thought the NOT EXISTS on the
"da_data_restrict_except_open" table was killing performance, but the
query took the exact same amount of time after I deleted all rows from
this table. Note that the hard-coded 999999999.0, and 4000 parameters,
as well as the parameter to svp_getparentproviders are the three
variables that change from one run of this query to the next.

I'm using Postgres 7.4.5 as packaged in Debian. shared_buffers is set
to 57344 and sort_mem=4096.

The machine has an AMD 1.8+ and ` gig of RAM. Here are some relevant
performance statistics:
richard:/usr/share/cups/model# cat /proc/sys/kernel/shmmax
536870912
richard:/usr/share/cups/model# cat /proc/sys/kernel/shmall
536870912
richard:/home/richard# hdparm -tT /dev/hda
Timing cached reads: 1112 MB in 2.00 seconds = 556.00 MB/sec
Timing buffered disk reads: 176 MB in 3.02 seconds = 58.28 MB/sec

I have included an EXPLAIN ANALYZE, relevant table counts, and relevant
indexing information. If anyone has any suggestions on how to improve
performance.... TIA!

SELECT tab.answer_id, client_id, question_id, recordset_id,
date_effective, virt_field_name
FROM
(
SELECT a.uid AS answer_id, a.client_id, a.question_id, recordset_id,
date_effective
FROM da_answer a
WHERE a.date_effective <= 9999999999.0
AND a.inactive != 1
AND
(
5000 = 4000
OR
(EXISTS (SELECT * FROM svp_getparentproviderids(1) WHERE
svp_getparentproviderids = a.provider_id))
)
UNION
SELECT a.uid AS answer_id, a.client_id, a.question_id, recordset_id,
date_effective
FROM da_answer a,
(
SELECT main_id
FROM da_data_restrict
WHERE type_id = 2
AND (provider_id IN (SELECT * FROM svp_getparentproviderids(1)))

UNION

SELECT sa.uid AS main_id
FROM da_answer sa
JOIN da_data_restrict_except_closed dr ON dr.main_id =
sa.uid AND dr.type_id = 2 AND dr.except_provider_id = 1
WHERE (restricted = 1)
AND (restricted_closed_except = 1)
AND sa.covered_by_roi = 1
UNION
SELECT sa.uid AS main_id
FROM da_answer sa
WHERE (restricted = 0)
AND (restricted_open_except = 1)
AND (NOT EXISTS (SELECT dr.main_id FROM
da_data_restrict_except_open dr WHERE (dr.main_id = sa.uid) AND
(dr.type_id = 2) AND (dr.except_provider_id in (select * from
svp_getparentproviderids(1)))))
AND sa.covered_by_roi = 1
UNION
SELECT sa.uid AS main_id FROM da_answer sa WHERE (restricted
= 0) AND (restricted_open_except = 0)
AND sa.covered_by_roi = 1
) sec
WHERE a.covered_by_roi = 1
AND a.date_effective <= 9999999999.0
AND a.inactive != 1
AND a.uid = sec.main_id
AND 5000 > 4000
) tab, da_question q
WHERE tab.question_id = q.uid AND (min_access_level <= 4000 OR
min_access_level IS NULL)

Table counts from relevant tables
da_question 1095
da_answer 21117
da_question 1095
da_data_restrict_except_closed 3087
da_data_restrict_except_open 13391
svp_getparentproviderids(1) 1

Relevant Index
create index in_da_data_restrict_provider_id on
da_data_restrict(provider_id);
create index in_da_data_restrict_main_id on da_data_restrict(main_id);
create index in_da_data_restrict_type_id on da_data_restrict(type_id);
create index in_da_data_restrict_client_id on
da_data_restrict(client_id);
create index in_da_dr_type_provider on
da_data_restrict(type_id,provider_id);

create index in_da_data_rec_provider_id ON
da_data_restrict_except_closed(provider_id);
create index in_da_data_rec_type_id ON
da_data_restrict_except_closed(type_id);
create index in_da_data_rec_main_id ON
da_data_restrict_except_closed(main_id);
create index in_da_data_rec_except_provider_id ON
da_data_restrict_except_closed(except_provider_id);

create index in_da_data_reo_provider_id ON
da_data_restrict_except_open(provider_id);
create index in_da_data_reo_type_id ON
da_data_restrict_except_open(type_id);
create index in_da_data_reo_main_id ON
da_data_restrict_except_open(main_id);
create index in_da_data_reo_except_provider_id ON
da_data_restrict_except_open(except_provider_id);

create index in_da_answer_client_id ON da_answer(client_id);
create index in_da_answer_provider_id ON da_answer(provider_id);
create index in_da_answer_question_id ON da_answer(question_id);
create index in_da_answer_recordset_id ON da_answer(recordset_id);
create index in_da_answer_restricted ON da_answer(restricted);
create index in_da_answer_restricted_open_except ON
da_answer(restricted_open_except);
create index in_da_answer_restricted_closed_except ON
da_answer(restricted_closed_except);
create index in_da_answer_date_effective ON da_answer(date_effective);
create index in_da_answer_inactive ON da_answer(inactive);
create index in_da_answer_covered_by_roi ON da_answer(covered_by_roi);

create index in_da_ed_inactive_roi ON da_answer(date_effective,inactive,
covered_by_roi);

create index in_da_question_mal ON da_question(min_access_level);