Re: Help Need some hindsight

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
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