From: Simon Riggs <simon@2ndQuadrant.com>
To: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Cc: List pgsql-patches <pgsql-patches@postgresql.org>
Subject: Re: WIP Join Removal
Date: Tue, 02 Sep 2008 13:33:34 +0100
Message-ID: <1220358814.4371.406.camel@ebony.2ndQuadrant> (raw)
In-Reply-To: <48BD2189.40400@enterprisedb.com>
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>
<48BD2189.40400@enterprisedb.com>
On Tue, 2008-09-02 at 14:20 +0300, Heikki Linnakangas wrote:
> 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)
You raise an interesting and important point that shows an error of
mine. Notice that
select a.col2 from a left outer join b on a.col1 = b.col1
*and* b.col2 = 1;
can be re-written as
select a.col2 from a;
whereas
select a.col2 from a left outer join b on a.col1 = b.col1
where b.col2 = 1;
cannot, as you show.
It seems I wrote my original tests using "and" instead of "where" and
hadn't noticed the distinction. Thanks for helping me catch that error.
I will put back the code that looks for an empty filter condition on the
checkrel. That day was not wasted after all.
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support
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To: pgsql-patches@postgresql.org
Cc: simon@2ndQuadrant.com, heikki.linnakangas@enterprisedb.com
Subject: Re: WIP Join Removal
In-Reply-To: <1220358814.4371.406.camel@ebony.2ndQuadrant>
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