Select on timestamp-day slower than timestamp alone

Hello all!

On PostgreSQL V7.3.2 on TRU64 I recognized the following phenomena
that a SELECT using a difference of a timestamp and an interval
in the WHERE clause does not use the index
but using a timestamp without a difference does use the index.
The semantic of both SELECT's is equal, i.e., the result is equal.

Therefore, the second way is much faster.

Any ideas?


In detail:
  table:

wetter=# \d wetter
               Table "public.wetter"
   Column   |           Type           | Modifiers
-----------+--------------------------+-----------
  sensor_id | integer                  | not null
  epoche    | timestamp with time zone | not null
  wert      | real                     | not null
Indexes: wetter_pkey primary key btree (sensor_id, epoche),
          wetter_epoche_idx btree (epoche),
          wetter_sensor_id_idx btree (sensor_id)
Triggers: RI_ConstraintTrigger_45702811,
           t_ins_wetter_wetterakt



Select not using index:
-----------------------
wetter=# explain select * from wetter where epoche between
'2003-05-06 06:50:54+00'::timestamp-'1 days'::interval
AND '2003-05-06 04:45:36';

          QUERY PLAN


---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Seq Scan on wetter  (cost=0.00..768644.57 rows=10253528 width=16)
    Filter: ((epoche >= ('2003-05-05 06:50:54'::timestamp without time
zone)::timestamp with time zone) AND (epoche <= '2003-05-06
04:45:36+00'::timestamp with time zone))
(2 rows)

wetter=#




Select using the index:
-----------------------
explain select * from wetter where epoche between '2003-05-05 06:50:54'
AND '2003-05-06 04:45:36';

QUERY PLAN


---------------------------------------------------------------------------------------------------------------------------------------------------
  Index Scan using wetter_epoche_idx on wetter  (cost=0.00..5.45 rows=1
width=16)
    Index Cond: ((epoche >= '2003-05-05 06:50:54+00'::timestamp with
time zone) AND (epoche <= '2003-05-06 04:45:36+00'::timestamp with time
zone))
(2 rows)

wetter=#




--

Mit freundlichen Gruessen / With best regards
    Reiner Dassing