7.3 vs 7.2 - different query plan, bad performance

Hi.

I have a problem with performance after upgrading from 7.2 to 7.3. Let's
see two simple tables:

CREATE TABLE a (
id integer,
parent_id integer
);

with 1632 records, and

CREATE TABLE b (
id integer
);

with 5281 records, and a litle more complex view:

CREATE VIEW v_c AS
SELECT t1.id,
(SELECT count(*) AS count FROM a t3 WHERE (t3.parent_id = t2.id)) AS children_count
FROM (b t1 LEFT JOIN a t2 ON ((t1.id = t2.id)));

Now see the query run under explain analyze:

Postgresql 7.2:

siaco=# explain analyze select count(*) from v_c;
NOTICE: QUERY PLAN:

Aggregate (cost=219.66..219.66 rows=1 width=8) (actual time=162.75..162.75 rows=1 loops=1)
-> Merge Join (cost=139.66..207.16 rows=5000 width=8) (actual time=95.07..151.46 rows=5281 loops=1)
-> Sort (cost=69.83..69.83 rows=1000 width=4) (actual time=76.18..82.37 rows=5281 loops=1)
-> Seq Scan on b t1 (cost=0.00..20.00 rows=1000 width=4) (actual time=0.02..22.02 rows=5281 loops=1)
-> Sort (cost=69.83..69.83 rows=1000 width=4) (actual time=18.86..25.38 rows=5281 loops=1)
-> Seq Scan on a t2 (cost=0.00..20.00 rows=1000 width=4) (actual time=0.02..6.70 rows=1632 loops=1)
Total runtime: 164.34 msec
EXPLAIN

Postgresql 7.3:

siaco=# explain analyze select count(*) from v_c;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=224.66..224.66 rows=1 width=8) (actual time=5691.77..5691.77 rows=1 loops=1)
-> Subquery Scan v_c (cost=139.66..212.16 rows=5000 width=8) (actual time=24.72..5687.77 rows=5281 loops=1)
-> Merge Join (cost=139.66..212.16 rows=5000 width=8) (actual time=24.72..5681.55 rows=5281 loops=1)
Merge Cond: ("outer".id = "inner".id)
-> Sort (cost=69.83..72.33 rows=1000 width=4) (actual time=18.82..21.09 rows=5281 loops=1)
Sort Key: t1.id
-> Seq Scan on b t1 (cost=0.00..20.00 rows=1000 width=4) (actual time=0.01..7.28 rows=5281 loops=1)
-> Sort (cost=69.83..72.33 rows=1000 width=4) (actual time=4.74..7.15 rows=5281 loops=1)
Sort Key: t2.id
-> Seq Scan on a t2 (cost=0.00..20.00 rows=1000 width=4) (actual time=0.02..2.13 rows=1632 loops=1)
SubPlan
-> Aggregate (cost=22.51..22.51 rows=1 width=0) (actual time=1.07..1.07 rows=1 loops=5281)
-> Seq Scan on a t3 (cost=0.00..22.50 rows=5 width=0) (actual time=0.80..1.06 rows=1 loops=5281)
Filter: (parent_id = $0)
Total runtime: 5693.62 msec
(15 rows)

I can't understand where comes the big difference in query plan from, and
(that's more important) - how to force postgres 7.3 to execute it more
efficient?

Notice, that both databases on both machines are identical and machine with
postgres 7.3 is even faster than the other one.

Richard.

--
"First they ignore you. Then they laugh at you. Then they
fight you. Then you win." - Mohandas Gandhi.