From: CN LIOU <cnliou@graffiti.net>
To: pgsql-sql@postgresql.org
Subject: Two Permance Questions
Date: Fri, 20 Sep 2002 10:07:11 +0800
Message-ID: <20020920020711.32693.qmail@graffiti.net> (raw)
Hi!
Q1. Is subquery better or join?
For subquery:
SELECT t1.c1,(SELECT t2.c2 FROM t2 WHERE t2.c1 = t1.c1) FROM t1
I wonder it will loop n times if t1 returns n rows. If this is the case, is it better to use join like this:
SELECT t1.c1,t2.c2 FROM t1,t2 WHERE t2.c1 = t1.c1
Q2. If the query is not optimize like this:
SELECT t1.c1,t2.c2 FROM t1,t2,t1,t2,t2 WHERE t2.c1=t1.c1 AND t1.c1=t2.c1 AND t1.c1=t2.c1
and the size of this clause can soar up to several kbytes, then can this query cause performance problem?
Regards,
--
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To: pgsql-sql@postgresql.org
Cc: cnliou@graffiti.net
Subject: Re: Two Permance Questions
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