Date: Wed, 31 Jul 2002 13:08:43 -0700 (PDT)
From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
To: Jinn Koriech <lists@idealint.co.uk>
Cc: PostgreSQL Mailing List <pgsql-sql@postgresql.org>
Subject: Re: negative queries puzzle
In-Reply-To: <1028142936.16285.44.camel@morph.magiclamp.2y.net>
Message-ID: <20020731130557.A19958-100000@megazone23.bigpanda.com>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII


On 31 Jul 2002, Jinn Koriech wrote:

> hi all,
>
> here's a query i've never been able to improve:
>
> i have an old data set and a new data set - in this case uk postcodes
> with eastings and northings.  i want to extract the new and changed
> postcodes from the new set.  to get the changed entries i use a join and
> it works okay:
>
> SELECT n.postcode, n.easting, n.northing FROM v_postcode_new n,
> v_postcode_old o WHERE n.postcode = o.postcode AND (n.easting <>
> o.lattitude OR n.northing <> o.longitude);
>
>
> but then to get the entirely new items out i use a sub query which takes
> for ever
>
> SELECT DISTINCT * FROM v_postcode_new WHERE postcode NOT IN ( SELECT
> postcode FROM v_postcode_old ) ORDER BY postcode ASC;
>
> does anyone know of a quicker way to accomplish this?  i guess there
> must be some cleaver way around it, but it's beyond me.

Hmm, a couple of possible other queries:

-- Do you really need the distinct?
select distinct * from v_postcode_new where not exists (
 select * from v_postcode_old where v_postcode_old.postcode=
                                    v_postcode_new.postcode);

Or maybe (just thought of this, think it should work, but
am not entirely sure)
select distinct v_postcode_new.* from
 v_postcode_new left outer join v_postcode_old using(postcode)
 where v_postcode_old.postcode is null;