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List:       postgresql-sql
Subject:    Re: [SQL] statement-level trigger sample out there?
From:       hubert depesz lubaczewski <depesz () depesz ! com>
Date:       2007-11-30 9:41:46
Message-ID: 20071130094146.GA10972 () depesz ! com
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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

-- 
quicksil1er: "postgres is excellent, but like any DB it requires a
highly paid DBA.  here's my CV!" :)
http://www.depesz.com/ - blog dla ciebie (i moje CV)

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