Date: Wed, 4 Aug 2010 13:48:08 +0200
From: "A. Kretschmer" <andreas.kretschmer@schollglas.com>
To: pgsql-sql@postgresql.org
Subject: Re: Help   Need some hindsight
Message-ID: <20100804114808.GB5435@a-kretschmer.de>
References: <4C5785D7.8040907@gmx.net>
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In response to Andreas :
>  Hi,
> I need to display log events (again).
> The log is simply like this
> log ( log_id serial primary key, create_ts timestamp default 
> localtimestamp, object_id, state_id, ....... )
> 
> It records the state of objects and when and what happend to to change 
> this state.
> 
> I'd like to get a list that shows the current state at any point of time 
> and the state of the last event before regarding the current object_id.
> The tricky bit is that both states should appear in the same row for 
> every row.
> 
> Help?


something like:

test=*# select * from log;
 id |             ts             | object_id | state_id
----+----------------------------+-----------+----------
  1 | 2010-08-04 13:24:19.648437 |         1 |        1
  2 | 2010-08-04 13:24:26.957629 |         1 |        2
  3 | 2010-08-04 13:24:38.883519 |         1 |        3
  4 | 2010-08-04 13:24:43.60719  |         1 |        2
  5 | 2010-08-04 13:24:51.123276 |         1 |        4
(5 rows)

test=*#
test=*#
test=*# 
select max(case when row_number=1 then id else null end) as current_state_id, 
       max(case when row_number=2 then id else null end) as last_state_id, 
       max(case when row_number=1 then state_id else null end) as current_state, 
       max(case when row_number=2 then state_id else null end) as last_state 
from ((select 1 as row_number, * from log where object_id=1 and ts < '2010-08-04 13:24:38.883520' order by ts desc limit 1) union all 
     (select 2, * fromlog where object_id=1 and ts < '2010-08-04 13:24:38.883520' order by ts desc limit 1 offset 1)) foo ;
 current_state_id | last_state_id | current_state | last_state
------------------+---------------+---------------+------------
                3 |             2 |             3 |          2
(1 row)

As you can see, the where-condition contains a timestamp that isn't in the table, it's after the id=3.

Maybe there are other, better solutions...


HTH, Andreas
-- 
Andreas Kretschmer
Kontakt:  Heynitz: 035242/47150,   D1: 0160/7141639 (mehr: -> Header)
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