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help / color / mirror / Atom feedFrom: Andreas Joseph Krogh <andreas@visena.com>
To: pgsql-sql@postgresql.org
Subject: Re: How to optimize WHERE column_a IS NOT NULL OR column_b = 'value'
Date: Mon, 11 Aug 2014 22:08:01 +0200 (CEST)
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Start a new thread På mandag 11. august 2014 kl. 21:49:32, skrev SENADIN <
senadin2006@yahoo.fr <mailto:senadin2006@yahoo.fr>>: Hello I am looking for a
funtion to calculate the size of a record in a table in a database. Thank you
Anned
Le Lundi 11 août 2014 14h37, Pavel Stehule <pavel.stehule@gmail.com> a
écrit : Hi 2014-08-11 18:01 GMT+02:00 Andreas Joseph Krogh <
andreas@visena.com <mailto:andreas@visena.com>>: Hi folks, I have the
following schema (simplified for this example). create table folder(id
integer primary key, name varchar not null); create table document(id serial
primary key, name varchar not null, owner_id integer not null, folder_id
integer references folder(id)); create index document_owner_idx ON
document(owner_id); create index document_folder_idx ON document(folder_id);
insert into folder(id, name) values(1, 'Folder A'); insert into folder(id,
name) values(2, 'Folder B'); insert into document(name, owner_id, folder_id)
values('Document A', 1, 1); insert into document(name, owner_id, folder_id)
values('Document B', 1, NULL); insert into document(name, owner_id, folder_id)
values('Document C', 2, 2); insert into document(name, owner_id, folder_id)
values('Document D', 2, NULL); select f.id <http://f.id/;, f.name
<http://f.name/;, doc.id <http://doc.id/;, doc.owner_id, doc.name
<http://doc.name/; FROM document doc left outer join folder f ON doc.folder_id =
f.id <http://f.id/; WHERE doc.folder_id is not null OR doc.owner_id = 1;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Nested Loop Left Join (cost=0.15..13.77 rows=4 width=76) (actual
time=0.031..0.045 rows=3 loops=1)
-> Seq Scan on document doc (cost=0.00..1.05 rows=4 width=44) (actual
time=0.012..0.018 rows=3 loops=1)
Filter: ((folder_id IS NOT NULL) OR (owner_id = 1))
Rows Removed by Filter: 1
-> Index Scan using folder_pkey on folder f (cost=0.15..3.17 rows=1
width=36) (actual time=0.005..0.006 rows=1 loops=3)
Index Cond: (doc.folder_id = id)
Planning time: 0.267 ms
Execution time: 0.094 ms
(8 rows) Is the a way to write a query which uses an index efficiently
for such a schema? I'd like to eliminate the Filter: ((folder_id IS NOT NULL)
OR (owner_id = 1)) and rather have "index cond" insted, is that possible?
your example is partially broken - ANALYZE and hashjoin and seqscan
penalization are missing - index scan is not used due too small table sizes
I tested 9.5, probably same as 9.4 and there indexes are used
postgres=# set enable_hashjoin to off;
SET
Time: 0.473 ms
postgres=# set enable_seqscan to off;
SET
Time: 0.904 ms
postgres=# explain select f.id <http://f.id/;, f.name <http://f.name/;, doc.id
<http://doc.id/;, doc.owner_id, doc.name <http://doc.name/;
FROM document doc left outer join folder f ON doc.folder_id = f.id
<http://f.id/;
WHERE doc.folder_id is not null OR doc.owner_id = 1;
QUERY
PLAN
────────────────────────────────────────────────────────────────────────────────────────────────
Merge Left Join (cost=0.26..24.38 rows=3 width=32)
Merge Cond: (doc.folder_id = f.id <http://f.id/;)
-> Index Scan using document_folder_idx on document doc
(cost=0.13..12.20 rows=3 width=23)
Filter: ((folder_id IS NOT NULL) OR (owner_id = 1))
-> Index Scan using folder_pkey on folder f (cost=0.13..12.16 rows=2
width=13)
Planning time: 0.663 ms
(6 rows)
default 9.2, 9.3, ...
postgres=# explain select f.id <http://f.id/;, f.name <http://f.name/;, doc.id
<http://doc.id/;, doc.owner_id, doc.name <http://doc.name/;
FROM document doc left outer join folder f ON doc.folder_id = f.id
<http://f.id/;
WHERE doc.folder_id is not null OR doc.owner_id = 1;
QUERY PLAN
─────────────────────────────────────────────────────────────────────
Hash Left Join (cost=1.04..2.12 rows=3 width=32)
Hash Cond: (doc.folder_id = f.id <http://f.id/;)
-> Seq Scan on document doc (cost=0.00..1.05 rows=3 width=23)
Filter: ((folder_id IS NOT NULL) OR (owner_id = 1))
-> Hash (cost=1.02..1.02 rows=2 width=13)
-> Seq Scan on folder f (cost=0.00..1.02 rows=2 width=13)
(6 rows)
and 9.2 after hashjoin and indexscan penalization
postgres=# explain select f.id <http://f.id/;, f.name <http://f.name/;, doc.id
<http://doc.id/;, doc.owner_id, doc.name <http://doc.name/;
FROM document doc left outer join folder f ON doc.folder_id = f.id
<http://f.id/;
WHERE doc.folder_id is not null OR doc.owner_id = 1;
QUERY
PLAN
────────────────────────────────────────────────────────────────────────────────────────────────
Merge Left Join (cost=0.00..24.62 rows=3 width=32)
Merge Cond: (doc.folder_id = f.id <http://f.id/;)
-> Index Scan using document_folder_idx on document doc
(cost=0.00..12.32 rows=3 width=23)
Filter: ((folder_id IS NOT NULL) OR (owner_id = 1))
-> Index Scan using folder_pkey on folder f (cost=0.00..12.28 rows=2
width=13)
(5 rows)
Time: 2.258 ms
What is your PostgreSQL?
Regards
Pavel
P.S. ten years ago I had a similar issue - "OR" predikates can be replaced
by UNION
you can try:
SELECT * FROM (SELECT * FROM doc
WHERE folder_id IS NOT NULL UNION SELECT * FROM doc WHERE owner_id = 1)
s LEFT JOIN folder ON s.folder_id = folder.id <http://folder.id/;
or some similar magic select f.id <http://f.id/;, f.name
<http://f.name/;, doc.id <http://doc.id/;, doc.owner_id, doc.name
<http://doc.name/; FROM document doc left outer join folder f ON doc.folder_id =
f.id <http://f.id/; WHERE doc.folder_id is not null OR doc.owner_id = 1;
--Andreas Joseph Krogh CTO / Partner - Visena AS Mobile: +47 909 56 963 <>
andreas@visena.com <mailto:andreas@visena.com> www.visena.com
<https://www.visena.com/; <https://www.visena.com/;
-- Andreas Joseph Krogh CTO / Partner - Visena AS Mobile: +47 909 56
963 andreas@visena.com <mailto:andreas@visena.com> www.visena.com
<https://www.visena.com; <https://www.visena.com;
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Subject: Re: How to optimize WHERE column_a IS NOT NULL OR column_b = 'value'
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