Thread: Forcing index usage without 'enable_hashjoin = FALSE'

Forcing index usage without 'enable_hashjoin = FALSE'

From
"Dan Langille"
Date:
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;

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



Re: Forcing index usage without 'enable_hashjoin = FALSE'

From
Chris
Date:
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?

--
Postgresql & php tutorials
http://www.designmagick.com/

Re: Forcing index usage without 'enable_hashjoin = FALSE'

From
"Dan Langille"
Date:
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



Re: Forcing index usage without 'enable_hashjoin = FALSE'

From
Tom Lane
Date:
"Dan Langille" <dan@langille.org> writes:
> Without leaving "enable_hashjoin = false", can you suggest a way to
> force the index usage?

Have you tried reducing random_page_cost?

FYI, 8.2 should be a bit better about this.

            regards, tom lane

Re: Forcing index usage without 'enable_hashjoin = FALSE'

From
"Dan Langille"
Date:
On 23 Aug 2006 at 22:30, Tom Lane wrote:

> "Dan Langille" <dan@langille.org> writes:
> > Without leaving "enable_hashjoin = false", can you suggest a way to
> > force the index usage?
>
> Have you tried reducing random_page_cost?

Yes.  No effect.

> FYI, 8.2 should be a bit better about this.

Good.  This query is not critical, but it would be nice.

Thank you.

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