DATE_PART() BUG? We have an SQL statement that is giving wrong output.

Peter Kelly (pkelly(at)ets(dot)net) reports a bug with a severity of 1
The lower the number the more severe it is.

Short Description
DATE_PART() BUG? We have an SQL statement that is giving wrong output.

Long Description
Here are operational details:

RedHat Linux 7.0:

Linux version 2.2.17-14 (root(at)porky(dot)devel(dot)redhat(dot)com) (gcc version egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)) #1 Mon Feb 5 15:25:12 EST 2001

PostgreSQL Version info:

postgresql-7.0.2-17
postgresql-server-7.0.2-17
postgresql-devel-7.0.2-17

tbs=# SELECT version();
version
-------------------------------------------------------------
PostgreSQL 7.0.2 on i686-pc-linux-gnu, compiled by gcc 2.96
(1 row)

Server/Memory Info:

Server is a Compaq Proliant 2500, Pentium Pro 200 with 256 MB RAM.

[root(at)lisa cgi-bin]# cat /proc/meminfo
total: used: free: shared: buffers: cached:
Mem: 263831552 151920640 111910912 82890752 47788032 74584064
Swap: 271556608 7589888 263966720
MemTotal: 257648 kB
MemFree: 109288 kB
MemShared: 80948 kB
Buffers: 46668 kB
Cached: 72836 kB
BigTotal: 0 kB
BigFree: 0 kB
SwapTotal: 265192 kB
SwapFree: 257780 kB

Sample Code
Here is an example of the whole statement:

SELECT
*, Date_Part('Month', CAST ('5/25/2001' AS DATE))
- Date_Part('Month', purchasedate ) + 1 AS thismonth
FROM customers
WHERE CAST('5/25/2001' AS DATE) - CAST ('12 months' AS INTERVAL) < purchasedate
AND purchasedate <= CAST ('5/25/2001' AS DATE)
AND 25 <= Date_Part('Day', purchasedate)
AND Date_Part('Day', purchasedate) <= 25
AND merchantnumber != 'odc12'
ORDER BY thismonth, id_num

Here is a simple example:

SELECT DATE_PART('DAY', CAST('04/1/2001' AS DATE)) as dayofmonth
dayofmonth
----------
31

Why does April 1st display as May 31st?

Here is the table def:

CREATE TABLE "customers" (
"id_num" int4 DEFAULT nextval('customers_id_num_seq'::text) NOT NULL,
"merchantnumber" character varying(9),
"producttype" character varying(7),
"purchasedate" date,
"emailaddress" character varying(60),
"firstname" character varying(25),
"lastname" character varying(50),
"customerfullname" character varying(40),
"salesordernumber" character varying(20),
"notes" character varying(80)
);

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