From: Piotr Gasidło <quaker@barbara.eu.org>
To: pgsql-performance@postgresql.org
Subject: Trouble with LEFT JOIN using VIEWS.
Date: Fri, 07 Dec 2007 11:55:08 +0100
Message-ID: <4759268C.6090508@barbara.eu.org> (raw)
Hello,
I've just hit problem, that is unusual for me.
quaker=> \d sites
Table "public.sites"
Column | Type | Modifiers
-----------+-------------------+----------------------------------------------------
id | integer | not null default
nextval('sites_id_seq'::regclass)
site_name | character varying | not null
user_id | integer | not null
extra | integer |
Indexes:
"sites_pkey" PRIMARY KEY, btree (id)
"sites_site_name_key_unique" UNIQUE, btree (site_name text_pattern_ops)
"sites_user_id_key" btree (user_id)
quaker=> \d users
Table "public.users"
Column | Type | Modifiers
-----------+-------------------+----------------------------------------------------
id | integer | not null default
nextval('users_id_seq'::regclass)
user_name | character varying | not null
extra | integer |
Indexes:
"users_pkey" PRIMARY KEY, btree (id)
Both tables filled with 100k records of random data. In users id is in
range from 1..100k, same in sites. In sites user_id is random, range
from 1..150k.
I've created views:
quaker=> \d users_secure
View "public.users_secure"
Column | Type | Modifiers
-----------+-------------------+-----------
id | integer |
user_name | character varying |
View definition:
SELECT users.id, users.user_name
FROM users;
quaker=> \d users_secure_with_has_extra
View "public.users_secure_with_has_extra"
Column | Type | Modifiers
-----------+-------------------+-----------
id | integer |
user_name | character varying |
has_extra | boolean |
View definition:
SELECT users.id, users.user_name, users.extra IS NOT NULL AS has_extra
FROM users;
Now, when I do simple query to find all data for sites matching
site_name like 'H3bh%' (there are at least one record in sites matching
this condition).
quaker=> explain analyze select s.site_name,u.user_name from
sites_secure s left join users_secure_with_has_extra u on u.id =
s.user_id where site_name like 'H3bh%' order by site_name limit 10;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=3897.02..3897.03 rows=2 width=44) (actual
time=430.326..430.331 rows=1 loops=1)
-> Sort (cost=3897.02..3897.03 rows=2 width=44) (actual
time=430.321..430.323 rows=1 loops=1)
Sort Key: sites.site_name
-> Nested Loop Left Join (cost=0.00..3897.01 rows=2
width=44) (actual time=290.103..430.301 rows=1 loops=1)
Join Filter: ("inner".id = "outer".user_id)
-> Index Scan using sites_site_name_key_unique on sites
(cost=0.00..6.01 rows=1 width=16) (actual time=0.044..0.054 rows=1
loops=1)
Index Cond: (((site_name)::text ~>=~ 'H3bh'::text)
AND ((site_name)::text ~<~ 'H3bi'::text))
Filter: ((site_name)::text ~~ 'H3bh%'::text)
-> Seq Scan on users (cost=0.00..1641.00 rows=100000
width=20) (actual time=0.007..245.406 rows=100000 loops=1)
Total runtime: 430.432 ms
(10 rows)
When I resign from LEFT JOIN users_secure_with_has_extra, and put JOIN
instead I've got:
quaker=> explain analyze select s.site_name,u.user_name from
sites_secure s join users_secure_with_has_extra u on u.id = s.user_id
where site_name like 'H3bh%' order by site_name limit 10;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=9.05..9.06 rows=1 width=24) (actual time=0.112..0.118
rows=1 loops=1)
-> Sort (cost=9.05..9.06 rows=1 width=24) (actual
time=0.106..0.108 rows=1 loops=1)
Sort Key: sites.site_name
-> Nested Loop (cost=0.00..9.04 rows=1 width=24) (actual
time=0.073..0.088 rows=1 loops=1)
-> Index Scan using sites_site_name_key_unique on sites
(cost=0.00..6.01 rows=1 width=16) (actual time=0.044..0.050 rows=1
loops=1)
Index Cond: (((site_name)::text ~>=~ 'H3bh'::text)
AND ((site_name)::text ~<~ 'H3bi'::text))
Filter: ((site_name)::text ~~ 'H3bh%'::text)
-> Index Scan using users_pkey on users
(cost=0.00..3.02 rows=1 width=16) (actual time=0.019..0.023 rows=1 loops=1)
Index Cond: (users.id = "outer".user_id)
Total runtime: 0.216 ms
(10 rows)
As explain shows PostgreSQL is using index scan on users, instead of seq
scan like in example above.
Now. When I use view with no has_extra field (important: field is a
simple function on extra field) I get expectable results. Both using
indexes.
quaker=> explain analyze select s.site_name,u.user_name from
sites_secure s left join users_secure u on u.id = s.user_id where
site_name like 'H3bh%' order by site_name limit 10;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=9.05..9.06 rows=1 width=24) (actual time=0.111..0.117
rows=1 loops=1)
-> Sort (cost=9.05..9.06 rows=1 width=24) (actual
time=0.105..0.107 rows=1 loops=1)
Sort Key: sites.site_name
-> Nested Loop Left Join (cost=0.00..9.04 rows=1 width=24)
(actual time=0.072..0.087 rows=1 loops=1)
-> Index Scan using sites_site_name_key_unique on sites
(cost=0.00..6.01 rows=1 width=16) (actual time=0.043..0.049 rows=1
loops=1)
Index Cond: (((site_name)::text ~>=~ 'H3bh'::text)
AND ((site_name)::text ~<~ 'H3bi'::text))
Filter: ((site_name)::text ~~ 'H3bh%'::text)
-> Index Scan using users_pkey on users
(cost=0.00..3.02 rows=1 width=16) (actual time=0.019..0.022 rows=1 loops=1)
Index Cond: (users.id = "outer".user_id)
Total runtime: 0.216 ms
(10 rows)
quaker=> explain analyze select s.site_name,u.user_name from
sites_secure s join users_secure u on u.id = s.user_id where site_name
like 'H3bh%' order by site_name limit 10;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=9.05..9.06 rows=1 width=24) (actual time=0.109..0.115
rows=1 loops=1)
-> Sort (cost=9.05..9.06 rows=1 width=24) (actual
time=0.104..0.106 rows=1 loops=1)
Sort Key: sites.site_name
-> Nested Loop (cost=0.00..9.04 rows=1 width=24) (actual
time=0.071..0.086 rows=1 loops=1)
-> Index Scan using sites_site_name_key_unique on sites
(cost=0.00..6.01 rows=1 width=16) (actual time=0.042..0.048 rows=1
loops=1)
Index Cond: (((site_name)::text ~>=~ 'H3bh'::text)
AND ((site_name)::text ~<~ 'H3bi'::text))
Filter: ((site_name)::text ~~ 'H3bh%'::text)
-> Index Scan using users_pkey on users
(cost=0.00..3.02 rows=1 width=16) (actual time=0.018..0.021 rows=1 loops=1)
Index Cond: (users.id = "outer".user_id)
Total runtime: 0.214 ms
(10 rows)
Why?
quaker=> select version();
version
-----------------------------------------------------------------------------------------------------------------------
PostgreSQL 8.1.10 on i486-pc-linux-gnu, compiled by GCC cc (GCC) 4.1.3
20070831 (prerelease) (Ubuntu 4.1.2-16ubuntu1)
(1 row)
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To: pgsql-performance@postgresql.org
Cc: quaker@barbara.eu.org
Subject: Re: Trouble with LEFT JOIN using VIEWS.
In-Reply-To: <4759268C.6090508@barbara.eu.org>
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