Thread: Guesses on what this NestLoop is for?

Folks,

I'm getting this plan on 7.2.4:

----------------------------------------------------------
explain
select events.event_id, events.event_name, type_name,
    COALESCE(cases.case_name || '(' || cases.docket || ')',
trial_groups.tgroup_name) as event_case,
    jw_date_format(events.event_date, events.event_tz, events.duration) as
show_date
FROM event_types, events
    LEFT OUTER JOIN cases ON (events.link_type = 'case' AND events.case_id =
cases.case_id)
    LEFT OUTER JOIN trial_groups ON ( events.link_type = 'tg' AND
        events.case_id = trial_groups.tgroup_id )
    LEFT OUTER JOIN event_history eh ON events.event_id = eh.event_id
WHERE events.status = 1 or events.status = 11
    and events.event_date > '2003-10-27'
    and events.etype_id = event_types.etype_id
    and (
        ( events.mod_user = 562 AND eh.event_id IS NULL )
        OR
        ( eh.mod_user = 562
          and not exists (select 1 from event_history eh2
              where eh2.event_id = eh.event_id
            and eh2.mod_date < eh.mod_date) )
        );

Nested Loop  (cost=100004949.08..2676373923.96 rows=3666858 width=197)
  ->  Hash Join  (cost=4949.08..8519.60 rows=43568 width=165)
        ->  Hash Join  (cost=4407.81..6615.02 rows=43568 width=149)
              ->  Hash Join  (cost=4403.21..6485.29 rows=43568 width=125)
                    ->  Seq Scan on events  (cost=0.00..1515.70 rows=43568
width=79)
                    ->  Hash  (cost=3108.07..3108.07 rows=115355 width=46)
                          ->  Seq Scan on cases  (cost=0.00..3108.07
rows=115355 width=46)
              ->  Hash  (cost=4.43..4.43 rows=143 width=24)
                    ->  Seq Scan on trial_groups  (cost=0.00..4.43 rows=143
width=24)
        ->  Hash  (cost=524.72..524.72 rows=13240 width=16)
              ->  Seq Scan on event_history eh  (cost=0.00..524.72 rows=13240
width=16)
  ->  Seq Scan on event_types  (cost=0.00..4.32 rows=106 width=32)
  SubPlan
    ->  Seq Scan on event_history eh2  (cost=0.00..557.82 rows=1 width=0)
-----------------------------------------------------------------

What I can't figure out is what is that inredibly expensive nested loop for?
If I could figure that out, maybe I could query around it.

Unfortunately, I can't EXPLAIN ANALYZE because the present query swamps the
machine, and it's a production server.  Also it never completes.

And yes, the system is vacuum full analyzed.   Event_history is under-indexed,
but the other tables are heavily indexed.

Ideas?

--
-Josh Berkus
 Aglio Database Solutions
 San Francisco

On Mon, 27 Oct 2003 15:32:41 -0800, Josh Berkus <josh@agliodbs.com>
wrote:
>FROM event_types, events
>    LEFT OUTER JOIN ...
>WHERE events.status = 1 or events.status = 11
>    and events.event_date > '2003-10-27'
>    and events.etype_id = event_types.etype_id
>    and ( ...
>        );
>
>
>What I can't figure out is what is that inredibly expensive nested loop for?

Sorry, I have no answer to your question, but may I ask whether you
really want to get presumably 106 output rows for each event with
status 1?

Or did you mean
     WHERE (events.status = 1 OR events.status = 11) AND ...

>Ideas?

I'd also try to push that NOT EXISTS condition into the FROM clause:

...LEFT JOIN (SELECT DISTINCT ON (event_id)
                     event_id, mod_date, mod_user
                FROM event_history
               ORDER BY event_id, mod_date
             ) AS eh ON (events.event_id = eh.event_id) ...
WHERE ...
  AND CASE WHEN eh.event_id IS NULL
           THEN events.mod_user
           ELSE eh.mod_user END = 562

If mod_user is NOT NULL in event_history, then CASE ... END can be
simplified to COALESCE(eh.mod_user, events.mod_user).

Servus
 Manfred

Manfred,

> Sorry, I have no answer to your question, but may I ask whether you
> really want to get presumably 106 output rows for each event with
> status 1?
>
> Or did you mean
>      WHERE (events.status = 1 OR events.status = 11) AND ...

Thanks!   I spent so much time tinkering around with the exists clauses, I
completely missed that.   Hopefully I'll get this client to upgrade to 7.4 so
that the explains will be more readable ....

--
Josh Berkus
Aglio Database Solutions
San Francisco