Re: Speeding up subselect ? - Mailing list pgsql-general

Is the situation possible where by you can run the subselect first at the
very start and put it into a variable.

Then put that variable in the query instead of the subselect

Darren

On Mon, 8 Jul 2002, Peter Alberer wrote:

> Hi there,
>
> i have the following query in a pl/pgsql procedure:
>
>     update lr_object_usage
>     set status = (case status
>         when ''OPEN_SUCC'' then ''CLOSED_SUCC''
>         when ''OPEN_FAIL'' then ''CLOSED_FAIL'' end)
>     where lr_object_usage_id in (select lr_object_usage_id from
> lr_locked_objects where context = p_exam_usage_id);
>
> the problem is the result of the subselect is obviously calculated for
> every row of lr_object_usage (30K rows) -> the update takes very long.
> Here is the explain output with p_exam_usage_id being 38191. Two simple
> selects with the result of the first one being used in the second one
> vs. the subselect. Divided into two selects
>
> Can I somehow tell the planer not to requery the subselect for every row
> ?
>
> Many TIA,
>
> peter
>
> --
> ------------------------------------------------------------------
>
> openacs=# select lr_object_usage_id from lr_locked_objects where context
> = 38191;
>  lr_object_usage_id
> --------------------
>               38192
>               38193
>               38194
>               38195
> (4 rows)
>
> openacs=# explain analyze  select lr_object_usage_id from
> lr_locked_objects where context = 38191;
> NOTICE:  QUERY PLAN:
>
> Seq Scan on lr_locked_objects  (cost=0.00..1.04 rows=1 width=4) (actual
> time=0.06..0.09 rows=4 loops=1)
> Total runtime: 0.18 msec
>
> EXPLAIN
>
> openacs=# select * from lr_object_usage where lr_object_usage_id in
> (38192,38193,38194,38195);
>  lr_object_usage_id | lr_object_id |          access_time          |
> user_id | status | context
> --------------------+--------------+-------------------------------+----
> -----+--------+---------
>               38192 |        13496 | 2002-07-08 17:47:01.001332+02 |
> 2434 |        |   38191
>               38193 |        13503 | 2002-07-08 17:47:01.001332+02 |
> 2434 |        |   38191
>               38194 |        13434 | 2002-07-08 17:47:01.001332+02 |
> 2434 |        |   38191
> (3 rows)
>
> openacs=# explain analyze select * from lr_object_usage where
> lr_object_usage_id in (38192,38193,38194,38195);
> NOTICE:  QUERY PLAN:
>
> Index Scan using lr_object_usage_lr_object_usage,
> lr_object_usage_lr_object_usage, lr_object_usage_lr_object_usage,
> lr_object_usage_lr_object_usage on lr_object_usage  (cost=0.00..12.18
> rows=4 width=39) (actual time=0.06..0.20 rows=3 loops=1)
> Total runtime: 0.38 msec
>
> EXPLAIN
>
> Doing the same thing with the subselect:
>
> openacs=# explain analyze select * from lr_object_usage where
> lr_object_usage_id in (select lr_object_usage_id from lr_locked_objects
> where context = 38191);
> NOTICE:  QUERY PLAN:
>
> Seq Scan on lr_object_usage  (cost=0.00..17860.59 rows=16514 width=39)
> (actual time=2640.91..2646.47 rows=3 loops=1)
>   SubPlan
>     ->  Seq Scan on lr_locked_objects  (cost=0.00..1.04 rows=1 width=4)
> (actual time=0.03..0.06 rows=4 loops=33080)
> Total runtime: 2646.60 msec
>
> EXPLAIN
>
>
>
>
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--
Darren Ferguson