From: Josh Berkus <josh@agliodbs.com>
To: pgsql-sql@postgresql.org
Subject: How can I tell the performance difference?
Date: Thu, 16 May 2002 14:29:50 -0700
Message-ID: <200205161429.50058.josh@agliodbs.com> (raw)
Folks,
I have a view which will be used very, very often in my database. As such, I
need to evaluate which of the two following structures is the best for that
view. However, I can't easily figure out which of the two explain plans
looks more costly. Can anybody help?
view #1:
create view juris as
select juris_id, juris_name, juris_code, notes,
min(treeno) as lnode, max(treeno) as rnode
from juris_desc JOIN juris_tree USING (juris_id)
group by juris_id, juris_name, juris_code, notes;
EXPLAIN:
Subquery Scan juris (cost=2.86..3.10 rows=2 width=70)
-> Aggregate (cost=2.86..3.10 rows=2 width=70)
-> Group (cost=2.86..3.02 rows=16 width=70)
-> Sort (cost=2.86..2.86 rows=16 width=70)
-> Hash Join (cost=1.20..2.54 rows=16 width=70)
-> Seq Scan on juris_desc (cost=0.00..1.08 rows=8
width=62)
-> Hash (cost=1.16..1.16 rows=16 width=8)
-> Seq Scan on juris_tree (cost=0.00..1.16
rows=16 width=8)
View #2:
create view juris2 as
select juris_id, juris_name, juris_code, notes,
lnode, rnode
from juris_desc JOIN
(SELECT juris_id, min(treeno) as lnode, max(treeno) as rnode
FROM juris_tree GROUP BY juris_id)
jt USING (juris_id);
EXPLAIN
Hash Join (cost=1.60..2.72 rows=1 width=74)
-> Seq Scan on juris_desc (cost=0.00..1.08 rows=8 width=62)
-> Hash (cost=1.60..1.60 rows=2 width=8)
-> Subquery Scan jt (cost=1.48..1.60 rows=2 width=8)
-> Aggregate (cost=1.48..1.60 rows=2 width=8)
-> Group (cost=1.48..1.52 rows=16 width=8)
-> Sort (cost=1.48..1.48 rows=16 width=8)
-> Seq Scan on juris_tree (cost=0.00..1.16
rows=16 width=8)
I'd very much like to evaluate this before the database goes into production.
Thanks!
--
-Josh Berkus
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Subject: Re: How can I tell the performance difference?
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