Date: Fri, 30 Nov 2007 10:41:46 +0100
From: hubert depesz lubaczewski <depesz@depesz.com>
To: Christian Kindler <christian.kindler@gmx.net>
Cc: Pavel Stehule <pavel.stehule@gmail.com>,
	Stephen Cook <sclists@gmail.com>, pgsql-sql@postgresql.org
Subject: Re: statement-level trigger sample out there?
Message-ID: <20071130094146.GA10972@depesz.com>
Reply-To: depesz@depesz.com
References: <20071129150454.99350@gmx.net>
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On Thu, Nov 29, 2007 at 04:04:54PM +0100, Christian Kindler wrote:
> I have a realy big table (> 2'000'000 records). every second there are several inserts and updates. the thing is i need a last row reference depending on a foreing_key. 
> 
> something like this:
> 
>  id, foreign_key, last_id, value1, value1, date
> >1<,           3,    null,     12,     13, 2007-01-01
> >2<,           4,    null,     11,     10, 2007-01-01
>   4,           3,     >1<,     12,     13, 2007-01-02
>   5,           4,     >2<,     11,     10, 2007-01-02
> ...
> 
> of course the sequence can have holes so I have to calculate the real last row id. for now i calculate for each row by invoking a "select max(id) where foreign_key = $1" but this cost a lot of performance. I could do this easily with one update for the whole query - if i could know which foreign_key and which date range was performed.


create index q on table (foreing_key, id);
and then:
select max(id) from table where foreing_key = ?;
should be very fast.
if it is not:
select id from table where foreing_key = ? order by foreing_key desc, id desc limit 1;

will be fast.

of course - remember about vacuum/analyze.

depesz

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