Skip site navigation (1) Skip section navigation (2)

Re: Query Plan far worse in 7.3.2 than 7.2.1

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Peter Darley" <pdarley(at)kinesis-cem(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Query Plan far worse in 7.3.2 than 7.2.1
Date: 2003-04-30 14:54:27
Message-ID: 22179.1051714467@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performancepgsql-sql
"Peter Darley" <pdarley(at)kinesis-cem(dot)com> writes:
> SELECT COUNT(*) FROM Border_Shop_List  WHERE NOT EXISTS (SELECT Foreign_Key=
>  FROM Sample WHERE Foreign_Key=3D'Quantum_' || Border_Shop_List.Assignment_=
> ID || '_' || Assignment_Year || '_' || Evaluation_ID)

What's the datatype of Foreign_Key?

I'm betting that it's varchar(n) or char(n).  The result of the ||
expression is text, and so the comparison can't use a varchar index
unless you explicitly cast it to varchar:
	WHERE Foreign_Key = ('Quantum_' || ... || Evaluation_ID)::varchar

I think 7.2 had some kluge in it that would allow a varchar index to be
used anyway, but we took out the kluge because it was semantically wrong
(it would also allow use of a char(n) index in place of a text
comparison, which alters the semantics...)

			regards, tom lane


In response to

Responses

pgsql-performance by date

Next:From: Peter DarleyDate: 2003-04-30 15:09:12
Subject: Re: Query Plan far worse in 7.3.2 than 7.2.1
Previous:From: Peter DarleyDate: 2003-04-30 14:39:24
Subject: Query Plan far worse in 7.3.2 than 7.2.1

pgsql-sql by date

Next:From: Peter DarleyDate: 2003-04-30 15:09:12
Subject: Re: Query Plan far worse in 7.3.2 than 7.2.1
Previous:From: Peter DarleyDate: 2003-04-30 14:39:24
Subject: Query Plan far worse in 7.3.2 than 7.2.1

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group