Re: Forcing index usage without 'enable_hashjoin = FALSE' - Mailing list pgsql-performance

On 23 Aug 2006 at 13:31, Chris wrote:

> Dan Langille wrote:
> > I'm using PostgreSQL 8.1.4 and I'm trying to force the planner to use
> > an index.  With the index, I get executions times of 0.5 seconds.
> > Without, it's closer to 2.5 seconds.
> >
> > Compare these two sets of results (also provided at
> > http://rafb.net/paste/results/ywcOZP66.html
> > should it appear poorly formatted below):
> >
> > freshports.org=# \i test2.sql
> >
> > QUERY PLAN
> > ----------------------------------------------------------------------
> > ----------------------------------------------------------------------
> > -
> >  Merge Join  (cost=24030.39..24091.43 rows=3028 width=206) (actual
> > time=301.301..355.261 rows=3149 loops=1)
> >    Merge Cond: ("outer".id = "inner".category_id)
> >    ->  Sort  (cost=11.17..11.41 rows=97 width=4) (actual
> > time=0.954..1.300 rows=95 loops=1)
> >          Sort Key: c.id
> >          ->  Seq Scan on categories c  (cost=0.00..7.97 rows=97
> > width=4) (actual time=0.092..0.517 rows=97 loops=1)
> >    ->  Sort  (cost=24019.22..24026.79 rows=3028 width=206) (actual
> > time=300.317..314.114 rows=3149 loops=1)
> >          Sort Key: p.category_id
> >          ->  Nested Loop  (cost=0.00..23844.14 rows=3028 width=206)
> > (actual time=0.082..264.459 rows=3149 loops=1)
> >                ->  Seq Scan on ports p  (cost=0.00..6141.11 rows=3028
> > width=206) (actual time=0.026..133.575 rows=3149 loops=1)
> >                      Filter: (status = 'D'::bpchar)
> >                ->  Index Scan using element_pkey on element e
> > (cost=0.00..5.83 rows=1 width=4) (actual time=0.022..0.026 rows=1
> > loops=3149)
> >                      Index Cond: ("outer".element_id = e.id)
> >  Total runtime: 369.869 ms
> > (13 rows)
> >
> > freshports.org=# set enable_hashjoin = true;
> > SET
> > freshports.org=# \i test2.sql
> >                                                            QUERY PLAN
> > ----------------------------------------------------------------------
> > ----------------------------------------------------------
> >  Hash Join  (cost=6156.90..13541.14 rows=3028 width=206) (actual
> > time=154.741..2334.366 rows=3149 loops=1)
> >    Hash Cond: ("outer".category_id = "inner".id)
> >    ->  Hash Join  (cost=6148.68..13472.36 rows=3028 width=206)
> > (actual time=153.801..2288.792 rows=3149 loops=1)
> >          Hash Cond: ("outer".id = "inner".element_id)
> >          ->  Seq Scan on element e  (cost=0.00..4766.70 rows=252670
> > width=4) (actual time=0.022..1062.626 rows=252670 loops=1)
> >          ->  Hash  (cost=6141.11..6141.11 rows=3028 width=206)
> > (actual time=151.105..151.105 rows=3149 loops=1)
> >                ->  Seq Scan on ports p  (cost=0.00..6141.11 rows=3028
> > width=206) (actual time=0.027..131.072 rows=3149 loops=1)
> >                      Filter: (status = 'D'::bpchar)
> >    ->  Hash  (cost=7.97..7.97 rows=97 width=4) (actual
> > time=0.885..0.885 rows=97 loops=1)
> >          ->  Seq Scan on categories c  (cost=0.00..7.97 rows=97
> > width=4) (actual time=0.076..0.476 rows=97 loops=1)
> >  Total runtime: 2346.877 ms
> > (11 rows)
> >
> > freshports.org=#
> >
> > Without leaving "enable_hashjoin = false", can you suggest a way to
> > force the index usage?
> >
> > FYI, the query is:
> >
> > explain analyse
> > SELECT P.id,
> >        P.category_id,
> >        P.version         as version,
> >        P.revision        as revision,
> >        P.element_id,
> >        P.maintainer,
> >        P.short_description,
> >        to_char(P.date_added - SystemTimeAdjust(), 'DD Mon YYYY
> > HH24:MI:SS') as date_added,
> >        P.last_commit_id  as last_change_log_id,
> >        P.package_exists,
> >        P.extract_suffix,
> >        P.homepage,
> >        P.status,
> >        P.broken,
> >        P.forbidden,
> >        P.ignore,
> >        P.restricted,
> >        P.deprecated,
> >        P.no_cdrom,
> >        P.expiration_date,
> >        P.latest_link
> >   FROM categories C, ports P JOIN element E on P.element_id = E.id
> >  WHERE P.status      = 'D'
> >    AND P.category_id = C.id;
> >
>
> I doubt it would make a difference but if you:
>
> ...
> FROM categories C JOIN ports P on P.category_id=C.id JOIN element E on
> P.element_id = E.id
> WHERE P.status      = 'D';
>
> does it change anything?

Not really, no:

freshports.org=# \i test3.sql

QUERY PLAN
----------------------------------------------------------------------
----------------------------------------------------------------------
---
 Hash Join  (cost=5344.62..12740.73 rows=3365 width=204) (actual
time=63.871..2164.880 rows=3149 loops=1)
   Hash Cond: ("outer".category_id = "inner".id)
   ->  Hash Join  (cost=5336.41..12665.22 rows=3365 width=204)
(actual time=62.918..2122.529 rows=3149 loops=1)
         Hash Cond: ("outer".id = "inner".element_id)
         ->  Seq Scan on element e  (cost=0.00..4767.58 rows=252758
width=4) (actual time=0.019..1024.299 rows=252791 loops=1)
         ->  Hash  (cost=5328.00..5328.00 rows=3365 width=204)
(actual time=60.228..60.228 rows=3149 loops=1)
               ->  Bitmap Heap Scan on ports p  (cost=34.02..5328.00
rows=3365 width=204) (actual time=1.900..41.316 rows=3149 loops=1)
                     Recheck Cond: (status = 'D'::bpchar)
                     ->  Bitmap Index Scan on ports_deleted
(cost=0.00..34.02 rows=3365 width=0) (actual time=1.454..1.454
rows=3149 loops=1)
                           Index Cond: (status = 'D'::bpchar)
   ->  Hash  (cost=7.97..7.97 rows=97 width=4) (actual
time=0.890..0.890 rows=97 loops=1)
         ->  Seq Scan on categories c  (cost=0.00..7.97 rows=97
width=4) (actual time=0.074..0.497 rows=97 loops=1)
 Total runtime: 2176.784 ms
(13 rows)

freshports.org=#



--
Dan Langille : Software Developer looking for work
my resume: http://www.freebsddiary.org/dan_langille.php