Thread: How can this be?

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.

Martin


-- Executing query:
SELECT count(*) from agent;
Total query runtime: 54 ms.
Data retrieval runtime: 31 ms.
1 rows retrieved.
Result: 5353

-- Executing query:
VACUUM ANALYZE agent;

-- 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;

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

On Fri, 16 Sep 2005, 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.

Are there any tables that reference agent or other triggers?  My first
guess would be that there's a foreign key check for something else that's
referencing agent.agent_id for which an index scan isn't being used.