On using "date 'XXX' + interval 'XXX'" vs "date 'XXX'"

Hello-

In rewriting some queries I noticed a huge performance penalty when
using a "date + interval" summation in the SELECT statement, versus a
single simple "date".  It is almost as though the "date + interval" is
being calculated for each row...

Much easier to demonstrate than it is to explain (note the runtimes of
the queries, 72ms versus 482ms):

===============================================
main_v0_8=# explain analyze select count(*) from sample_tracker where
initdate>=date '2002-02-01';
NOTICE:  QUERY PLAN:

Aggregate  (cost=607.24..607.24 rows=1 width=0) (actual
time=72.08..72.08 rows=1 loops=1)
   ->  Seq Scan on sample_tracker  (cost=0.00..595.74 rows=4600 width=0)
(actual time=0.04..63.62 rows=4266 loops=1)
Total runtime: 72.20 msec

EXPLAIN

===============================================
main_v0_8=# explain analyze select count(*) from sample_tracker where
initdate>=date '2002-01-01' + interval '1 month';
NOTICE:  QUERY PLAN:

Aggregate  (cost=738.23..738.23 rows=1 width=0) (actual
time=482.49..482.49 rows=1 loops=1)
   ->  Seq Scan on sample_tracker  (cost=0.00..723.98 rows=5700 width=0)
(actual time=0.13..470.94 rows=4266 loops=1)
Total runtime: 482.62 msec

EXPLAIN

===============================================
main_v0_8=# explain ANALYZE select date '2002-01-01' + interval '1 month';
NOTICE:  QUERY PLAN:

Result  (cost=0.00..0.01 rows=1 width=0) (actual time=0.04..0.05 rows=1
loops=1)
Total runtime: 0.09 msec

===============================================
main_v0_8=# select version();
                            version
-------------------------------------------------------------
  PostgreSQL 7.2.1 on i686-pc-linux-gnu, compiled by GCC 2.96



Seems like this could be something ripe for optimization...

--

-**-*-*---*-*---*-*---*-----*-*-----*---*-*---*-----*-----*-*-----*---
  Jon Lapham
  Extracta Moléculas Naturais, Rio de Janeiro, Brasil
  email: lapham@extracta.com.br      web: http://www.extracta.com.br/
***-*--*----*-------*------------*--------------------*---------------