constraint with reference to the same table

Hello.

I'm using PostgreSQL 7.3.1 on i686-pc-linux-gnu, compiled by GCC egcs-2.91.66.

Here is topic. Table transactions:

=> \d transactions
Table "public.transactions"
Column | Type | Modifiers
-------------+--------------+-----------
trxn_id | integer | not null
trxn_ret | integer |
trxn_for | integer |
status | numeric(2,0) | not null
auth_status | numeric(2,0) | not null
Indexes: transactions_pkey primary key btree (trxn_id)
Foreign Key constraints: trxns_id FOREIGN KEY (trxn_id) REFERENCES connections(conn_id) ON UPDATE NO ACTION ON DELETE NO ACTION,
trxns_ret FOREIGN KEY (trxn_ret) REFERENCES transactions(trxn_id) ON UPDATE NO ACTION ON DELETE NO ACTION,
trxns_for FOREIGN KEY (trxn_for) REFERENCES transactions(trxn_id) ON UPDATE NO ACTION ON DELETE NO ACTION

As you can see, trxns_ret and trxns_for constraints references to the same table they come from.

Maintenance of system includes the following step:
delete from transactions where transactions.trxn_id = uneeded_trxns.trxn_id;
transactions volume is about 10K-20K rows.
uneeded_trxns volume is about 3K-5K rows.

Problem: It takes to MUCH time. EXPLAIN says:
=> explain delete from transactions where transactions.trxn_id = balance_delete_data.conn_id;
QUERY PLAN
-----------------------------------------------------------------------------------
Hash Join (cost=86.47..966.66 rows=5238 width=14)
Hash Cond: ("outer".trxn_id = "inner".conn_id)
-> Seq Scan on transactions (cost=0.00..503.76 rows=24876 width=10)
-> Hash (cost=73.38..73.38 rows=5238 width=4)
-> Seq Scan on balance_delete_data (cost=0.00..73.38 rows=5238 width=4)
(5 rows)

I was waiting for about 30 minutes and then hit ^C.

After some time spent dropping indexes and constraints, I've found out, that problem was in
those 2 "cyclic" constraints. After drop, query passed in some seconds (that is suitable).

Question: why so?
Thanks in advance.

--

Victor Yegorov