From: | Ksenia Marasanova <ksenia(dot)marasanova(at)gmail(dot)com> |
---|---|
To: | pgsql-performance(at)postgresql(dot)org |
Subject: | LEFT JOIN optimization |
Date: | 2005-09-11 17:12:58 |
Message-ID: | 130df19305091110125a2d174f@mail.gmail.com |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-performance |
Hi list,
I don't have much experience with Postgres optimization, somehow I was
happily avoiding anything more difficult than simple select statement,
and it was working all right.
Now LEFT JOIN must be used, and I am not happy with the performance:
It takes about 5 seconds to run very simple LEFT JOIN query on a table
"user_" with ~ 13.000 records left joined to table "church" with ~ 300
records on Powerbook PPC 1.67 GHz with 1.5 GB ram.
Is it normal?
Some details:
test=# explain select * from user_ left join church on user_.church_id
= church.id;
QUERY PLAN
---------------------------------------------------------------------
Hash Left Join (cost=6.44..7626.69 rows=12763 width=325)
Hash Cond: ("outer".church_id = "inner".id)
-> Seq Scan on user_ (cost=0.00..7430.63 rows=12763 width=245)
-> Hash (cost=5.75..5.75 rows=275 width=80)
-> Seq Scan on church (cost=0.00..5.75 rows=275 width=80)
(5 rows)
From what I understand, it doesn't use foreign key index on user_
table. So I tried:
mydb=# set enable_seqscan='false';
SET
mydb=# explain select * from user_ left join church on user_.church_id
= church.id;
QUERY PLAN
---------------------------------------------------------------------------------------------
Merge Right Join (cost=0.00..44675.77 rows=12763 width=325)
Merge Cond: ("outer".id = "inner".church_id)
-> Index Scan using chirch_pkey on church (cost=0.00..17.02
rows=275 width=80)
-> Index Scan using user__church_id on user_ (cost=0.00..44500.34
rows=12763 width=245)
(4 rows)
It's my first time reading Query plans, but from wat I understand, it
doesn't make the query faster..
Any tips are greatly appreciated.
--
Ksenia
From | Date | Subject | |
---|---|---|---|
Next Message | Stephen Frost | 2005-09-11 21:00:36 | Re: LEFT JOIN optimization |
Previous Message | John A Meinel | 2005-09-11 12:35:53 | Re: shared buffers |