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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