To: "Henry B. Hotz" <hotz@jpl.nasa.gov>
Cc: Stefan Weiss <spaceman-4b9f8-20030703@ausgehaucht.sensenmann.at>,
	pgsql-docs@postgresql.org
Subject: Re: INNER JOINS in sql-select.html 
In-reply-to: <p05210602bbcc8fb0d68c@[137.78.212.225]> 
References: <1503129.DJSjvup6Wb@weyoun.foo.at> <3687.1067554564@sss.pgh.pa.us>
	<p05210602bbcc8fb0d68c@[137.78.212.225]>
Comments: In-reply-to "Henry B. Hotz" <hotz@jpl.nasa.gov>
	message dated "Mon, 03 Nov 2003 15:08:05 -0800"
Date: Mon, 03 Nov 2003 19:38:21 -0500
Message-ID: <17695.1067906301@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Henry B. Hotz" <hotz@jpl.nasa.gov> writes:
> You can imply the issue without obfuscating things.  How about:

> A CROSS JOIN or INNER JOIN is a simple Cartesian product, the same 
> as you get from listing the two items at the top level of FROM. 
> CROSS JOIN yields the same results as INNER JOIN ON (TRUE), that is, 
> no rows are removed by qualification.

Okay, but that doesn't do the trick --- it implies that CROSS JOIN isn't
equivalent to INNER JOIN ON (TRUE), when in fact they are equivalent,
both as to result and performance characteristics.  The issue at hand is
that an explicit "a JOIN b" may not be equivalent to "FROM a, b".

I reworded the passage as

        CROSS JOIN and INNER JOIN
        produce a simple Cartesian product, the same result as you get from
        listing the two items at the top level of FROM,
	but restricted by the join condition (if any).
        CROSS JOIN is equivalent to INNER JOIN ON
        (TRUE), that is, no rows are removed by qualification.

does that help?

			regards, tom lane