index / sequential scan problem

Hi all,

Im currently taking my first steps with db optimizations and am wondering
whats happening here and if/how i can help pg choose the better plan.

Thanks,
Fabian

>>>

psql (PostgreSQL) 7.2.2

perg_1097=# VACUUM ANALYZE ;
VACUUM
perg_1097=# EXPLAIN ANALYZE select notiz_id, obj_id, obj_typ
perg_1097-# from notiz_objekt a
perg_1097-# where not exists
perg_1097-# (
perg_1097(# select 1
perg_1097(# from notiz_gelesen b
perg_1097(# where ma_id = 2001
perg_1097(# and ma_pid = 1097
perg_1097(# and a.notiz_id = b.notiz_id
perg_1097(# )
perg_1097-# ;
NOTICE: QUERY PLAN:

Seq Scan on notiz_objekt a (cost=0.00..56125.80 rows=15561 width=12)
(actual time=0.28..2305.52 rows=31122 loops=1)
SubPlan
-> Seq Scan on notiz_gelesen b (cost=0.00..1.79 rows=1 width=0)
(actual time=0.07..0.07 rows=0 loops=31122)
Total runtime: 2334.42 msec

EXPLAIN
perg_1097=# SET enable_seqscan to false;
SET VARIABLE
perg_1097=# EXPLAIN ANALYZE select notiz_id, obj_id, obj_typ
perg_1097-# from notiz_objekt a
perg_1097-# where not exists
perg_1097-# (
perg_1097(# select 1
perg_1097(# from notiz_gelesen b
perg_1097(# where ma_id = 2001
perg_1097(# and ma_pid = 1097
perg_1097(# and a.notiz_id = b.notiz_id
perg_1097(# )
perg_1097-# ;
NOTICE: QUERY PLAN:

Seq Scan on notiz_objekt a (cost=100000000.00..100111719.36 rows=15561
width=12) (actual time=0.24..538.86 rows=31122 loops=1)
SubPlan
-> Index Scan using idx_notiz_gelesen_2 on notiz_gelesen
b (cost=0.00..3.57 rows=1 width=0) (actual time=0.01..0.01 rows=0 loops=31122)
Total runtime: 570.75 msec

EXPLAIN
perg_1097=#

perg_1097=# \d notiz_objekt;
Table "notiz_objekt"
Column | Type | Modifiers
----------+---------+-----------
notiz_id | integer |
obj_id | integer |
obj_typ | integer |
Indexes: idx_notiz_objekt_1,
idx_notiz_objekt_2

perg_1097=# \d notiz_gelesen;
Table "notiz_gelesen"
Column | Type | Modifiers
----------+--------------------------+----------------------------------------------------
notiz_id | integer |
ma_id | integer |
ma_pid | integer |
stamp | timestamp with time zone | default ('now'::text)::timestamp(6)
with time zone
anzeigen | character varying |
Indexes: idx_notiz_gelesen_1,
idx_notiz_gelesen_2

perg_1097=#

perg_1097=# select count(*) from notiz_objekt;
count
-------
31122
(1 row)

perg_1097=# select count(*) from notiz_gelesen;
count
-------
45
(1 row)

perg_1097=#

idx_notiz_gelesen_1 (ma_id,ma_pid)
idx_notiz_gelesen_2 (notiz_id)