Date: Mon, 5 Jun 2006 10:04:25 -0500
From: "Jim C. Nasby" <jnasby@pervasive.com>
To: Michael Artz <mlartz@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Bulk loading/merging
Message-ID: <20060605150425.GS53487@pervasive.com>
References: <ce4072df0605261148g4ec22267gc54b6b445e47c855@mail.gmail.com>
	<20060531010922.GC59464@pervasive.com>
	<e9c163070606011104w1d0cf7a6jca8cf5b36f2d8de3@mail.gmail.com>
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On Thu, Jun 01, 2006 at 02:04:46PM -0400, Michael Artz wrote:
> On 5/30/06, Jim C. Nasby <jnasby@pervasive.com> wrote:
> 
> >Your best bet is to do this as a single, bulk operation if possible.
> >That way you can simply do an UPDATE ... WHERE EXISTS followed by an
> >INSERT ... SELECT ... WHERE NOT EXISTS.
> 
> 
> 
> hmm, I don't quite understand what you are saying and I think my
> basic misunderstanding is how to use the UPDATE ... WHERE EXISTS to merge
> data in bulk.  Assuming that I bulk COPYed the data into a temporary
> table, I'd need to issue an UPDATE for each row in the newly created table,
> right?
> 
> For example, for a slightly different key,count schema:
> 
> CREATE TABLE kc (key integer, count integer);
> 
> and wanting to merge the following data by just updating the count for a
> given key to the equivalent of OLD.count + NEW.count:
> 
> 1,10
> 2,15
> 3,45
> 1,30
> 
> How would I go about using UPDATE ... WHERE EXISTS to update the "master" kc
> table from a (temporary) table loaded with the above data?

CREATE TEMP TABLE moo () LIKE kc;
COPY ... moo;
BEGIN;
    UPDATE kc
        SET count=kc.count + moo.count
        FROM moo
        WHERE moo.key = kc.key
    ;
    INSERT INTO kc(key, count)
        SELECT key, count
            FROM moo
            WHERE NOT EXISTS (
                    SELECT 1
                        FROM kc
                        WHERE kc.key = moo.key
                    )
    ;
COMMIT;
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461