7.4 - FK constraint performance

PostgreSQL 7.4 on i386-linux-gnu, compiled by GCC 2.96

-- about 10 records
CREATE TABLE my.Small
( id my.dint NOT NULL, code my.dvalue NOT NULL, CONSTRAINT pk_1 PRIMARY KEY (id),
) WITHOUT OIDS;

-- about 80M rows
CREATE TABLE my.Large 
( id my.dlong NOT NULL,  small_id my.dint NOT NULL, value my.value, CONSTRAINT pk_2 PRIMARY KEY (id), CONSTRAINT fk_2
FOREIGNKEY (small_id) REFERENCES my.small (id) ON UPDATE
 
RESTRICT ON DELETE RESTRICT,
) WITHOUT OIDS;

CREATE INDEX small_fk  ON my.Large USING btree (small_id);

---------------------------------------------
The fowllowing queiries run in less than 40 ms.
1) select 1 from Large where small_id = 239
2) SELECT 1 FROM ONLY "my"."Large" x WHERE "small_id" = 239 FOR UPDATE OF x

3) delete from Small where id = 239 
Feb 11 18:07:36 srv07 postgres[2091]: [91-1] LOG:  statement: SELECT 1 FROM
ONLY "my"."Large" x WHERE "small_id" = $1 FOR UPDATE OF x Feb 11 18:10:41 srv07

postgres[2091]: [92-1] LOG: duration: 185273.262 ms

When I try to delete record, it takes > 3 min. Why is it taking so long if 
practically the same select query (see (2)) is running very quickly. Anything
that can be done to fix it?

Thanks





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