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From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: List pgsql-patches <pgsql-patches@postgresql.org>
Subject: Re: WIP Join Removal
Date: Tue, 02 Sep 2008 14:20:41 +0300
Message-ID: <48BD2189.40400@enterprisedb.com> (raw)
In-Reply-To: <1220354413.4371.365.camel@ebony.2ndQuadrant>
References: <1220176372.4371.118.camel@ebony.2ndQuadrant>
	<48BC4127.30706@enterprisedb.com>
	<1220348487.4371.307.camel@ebony.2ndQuadrant>
	<48BD1374.4030504@enterprisedb.com>
	<1220352112.4371.349.camel@ebony.2ndQuadrant>
	<48BD1852.1070904@enterprisedb.com>
	<1220352898.4371.355.camel@ebony.2ndQuadrant>
	<48BD1D7C.6090002@enterprisedb.com>
	<1220354413.4371.365.camel@ebony.2ndQuadrant>

Simon Riggs wrote:
> select a.col2
> from a left outer join b on a.col1 = b.col1
> where b.col2 = 1;
> 
> is logically equivalent to 
> 
> select a.col2
> from a;

No, it's not:

postgres=# CREATE TABLE a (col1 int4, col2 int4);
CREATE TABLE
postgres=# CREATE TABLE b (col1 int4, col2 int4);
CREATE TABLE
postgres=# INSERT INTO a VALUES (1,1);
INSERT 0 1
postgres=# select a.col2 from a;
  col2
------
     1
(1 row)

postgres=# select a.col2 from a left outer join b on a.col1 = b.col1 
where b.col2 = 1;
  col2
------
(0 rows)

But anyway, Greg's example looks valid, and proves the point that 
removing a join isn't always a win.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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