In response to A. Kretschmer :
> > please, suggest an idea how to implement this in SQL without writing a
> > procedure.
> 
> There are a really nice additional contrib module from Jeff Davis,
> described here:
> 
> http://thoughts.j-davis.com/2010/03/09/temporal-postgresql-roadmap/
> 

short example, with your data:

test=# select * from rel_a;
 id | start_date |  end_date  |                         p
----+------------+------------+----------------------------------------------------
  1 | 01.01.2010 | 01.02.2010 | [01.01.2010 00:00:00 CET, 01.02.2010 00:00:00 
CET)
  2 | 03.02.2010 | 04.03.2010 | [03.02.2010 00:00:00 CET, 04.03.2010 00:00:00 
CET)
(2 rows)

test=# select * from rel_a where contains(p, period('15.01.2010'::date, 
'15.02.2010'::date));
 id | start_date | end_date | p
----+------------+----------+---
(0 rows)

test=# select * from rel_a where contains(p, period('15.01.2010'::date, 
'25.01.2010'::date));
 id | start_date |  end_date  |                         p
----+------------+------------+----------------------------------------------------
  1 | 01.01.2010 | 01.02.2010 | [01.01.2010 00:00:00 CET, 01.02.2010 00:00:00 
CET)
(1 row)


The column p is created with:

test=# alter table rel_a add column p period;

and

test=# update rel_a set p = period(start_date, end_date);


Andreas
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Andreas Kretschmer
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