From: Colin Fox <cfox@cfconsulting.ca>
To: pgsql-sql@postgresql.org
Subject: Efficiency Question
Date: Mon, 09 Sep 2002 00:13:04 GMT
Message-ID: <pan.2002.09.09.00.12.58.87388@cfconsulting.ca> (raw)
Hi, all.
I'm putting together a small query that should return the most recent
entry from a table by date (I can't use an ID field as new entries may be
added with older dates). It's not crucial that this run at 100%
efficiency, but I'm interested in the results and/or discussion, as this
will probably relate to other larger queries later.
Two methods occur to me, and I don't have a large enough data set to get
any kind of idea of the actual response, and I'm not sure I understand the
explain plan.
Method 1 is like this:
select
*
from
motm
where
creation_date = (select max(creation_date) from motm);
Which requires a subselect and a max operator on a date field. Method two
is:
select
*
from
motm
order by
creation_date desc
limit 1;
So in the first case I select the record that has the largest date. In the
second case, I order all the records, and then return only one.
Here's the explain for both:
----------
Method 1:
NOTICE: QUERY PLAN:
Merge Join (cost=23.77..23.96 rows=1 width=60)
InitPlan
-> Aggregate (cost=22.50..22.50 rows=1 width=8)
-> Seq Scan on motm (cost=0.00..20.00 rows=1000 width=8)
-> Sort (cost=22.67..22.67 rows=10 width=20)
-> Seq Scan on motm m (cost=0.00..22.50 rows=10 width=20)
-> Sort (cost=1.11..1.11 rows=5 width=40)
-> Seq Scan on people p (cost=0.00..1.05 rows=5 width=40)
EXPLAIN
----------
Method 2:
NOTICE: QUERY PLAN:
Limit (cost=84.91..84.91 rows=1 width=68)
-> Sort (cost=84.91..84.91 rows=50 width=68)
-> Merge Join (cost=70.94..83.50 rows=50 width=68)
-> Sort (cost=69.83..69.83 rows=1000 width=28)
-> Seq Scan on motm m (cost=0.00..20.00 rows=1000 width=28)
-> Sort (cost=1.11..1.11 rows=5 width=40)
-> Seq Scan on people p (cost=0.00..1.05 rows=5 width=40)
EXPLAIN
--------------
According to the cost score, it seems that method 1 is faster, almost 4x!
Is that actually the case?
Opinions welcome. :)
Colin
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To: pgsql-sql@postgresql.org
Cc: cfox@cfconsulting.ca
Subject: Re: Efficiency Question
In-Reply-To: <pan.2002.09.09.00.12.58.87388@cfconsulting.ca>
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