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

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?

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