Should casting to integer produce same result as trunc()

Hi all,

Had to squash timestamps to the nearest 5 minutes and things went wrong.

My simple understanding of trunc() and casting to an integer says that
there is a bug here.
Expect it is my understanding though.
Can someone set me straight?

And thank you all for a wonderfull RDBMS.

Allan


select version();
                                       version

------------------------------------------------------------------------
-------------
 PostgreSQL 8.2.4 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 4.1.0
(SUSE Linux)
(1 row)

-- trying to squash timestamps to five minutes. The result should be
different. Use casting.
select timestamp without time zone 'epoch' + (((extract( epoch from
'2011-08-22 08:37:30'::timestamp ) + 10 * 3600) / 300 )::integer) * 300
* INTERVAL '1
 second', timestamp without time zone 'epoch' + (((extract( epoch from
'2011-08-22 08:42:30'::timestamp ) + 10 * 3600) / 300 )::integer) * 300
* INTERVAL
 '1 second';
      ?column?       |      ?column?
---------------------+---------------------
 2011-08-22 08:40:00 | 2011-08-22 08:40:00
(1 row)


-- The result is different, correct. Use trunc().
select timestamp without time zone 'epoch' + trunc(((extract( epoch from
'2011-08-22 08:37:30'::timestamp ) + 10 * 3600) / 300 )) * 300 *
INTERVAL '1 sec
ond', timestamp without time zone 'epoch' + trunc(((extract( epoch from
'2011-08-22 08:42:30'::timestamp ) + 10 * 3600) / 300 )) * 300 *
INTERVAL '1 seco
nd';
      ?column?       |      ?column?
---------------------+---------------------
 2011-08-22 08:35:00 | 2011-08-22 08:40:00
(1 row)


-- Raw seconds. Different as expected.
select (((extract( epoch from '2011-08-22 08:37:30'::timestamp ) + 10 *
3600) )::integer), (((extract( epoch from '2011-08-22
08:42:30'::timestamp ) + 10
 * 3600) )::integer);
    int4    |    int4
------------+------------
 1314002250 | 1314002550
(1 row)


-- should be different but are not.
select (((extract( epoch from '2011-08-22 08:37:30'::timestamp ) + 10 *
3600) / 300 )::integer), (((extract( epoch from '2011-08-22
08:42:30'::timestamp
) + 10 * 3600) / 300 )::integer);
  int4   |  int4
---------+---------
 4380008 | 4380008
(1 row)


select (1314002250 / 300)::integer, (1314002550 / 300)::integer;
  int4   |  int4
---------+---------
 4380007 | 4380008
(1 row)


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