Re: How can this be?

On Fri, Sep 16, 2005 at 08:34:14PM -0500, Martin Nickel wrote:

> Hello all,
> Mostly Postgres makes sense to me. But now and then it does something
> that boggles my brain. Take the statements below. I have a table
> (agent) with 5300 rows. The primary key is agent_id. I can do SELECT
> agent_id FROM agent and it returns all PK values in less than half a
> second (dual Opteron box, 4G ram, SATA Raid 10 drive system).
>
> But when I do a DELETE on two rows with an IN statement, using the primary
> key index (as stated by EXPLAIN) it take almost 4 minutes.
> pg_stat_activity shows nine other connections, all idle.
>
> If someone can explain this to me it will help restore my general faith in
> order and consistancy in the universe.

When you delete a row from agent PG needs to find any matching rows in
office. Is office large? Is office(office_id) indexed?

> -- Executing query:
> DELETE FROM agent WHERE agent_id IN (15395, 15394);
> Query returned successfully: 2 rows affected, 224092 ms execution time.
>
> -- Executing query:
> EXPLAIN DELETE FROM agent WHERE agent_id IN (15395, 15394);
> Index Scan using agent2_pkey, agent2_pkey on agent (cost=0.00..7.27
> rows=2 width=6)
> Index Cond: ((agent_id = 15395) OR (agent_id = 15394))
>
> Here's my table
> CREATE TABLE agent
> (
> agent_id int4 NOT NULL DEFAULT nextval('agent_id_seq'::text),
> office_id int4 NOT NULL,
> lastname varchar(25),
> firstname varchar(25),
> ...other columns...
> CONSTRAINT agent2_pkey PRIMARY KEY (agent_id),
> CONSTRAINT agent_office_fk FOREIGN KEY (office_id) REFERENCES office (office_id) ON UPDATE RESTRICT ON DELETE RESTRICT
> )
> WITHOUT OIDS;

Cheers,
Steve