Inconsistant SQL results - Suspected error with query planing or query optimisation.

Hay all :)

Firstly, just wanted to say how much I appreceate all the hard work that
has gorn into postgres. It really is a fine system.

Anyway, mybug: I have a test SELECT statement (Listing A - see "sql
listings.txt") wich produces different results under two simular setups
(Listing B and Listing C). Each setup should product the same result for
the given SELECT statement.

In the first setup (Listing B) the table "items" has 10,000 records id'ed
from 1 to 10,000. The field "number" for every record except two records
(id:500 and 600) has the value NULL., the two exceptions (id: 500 and 600)
have the value 1.
This first setup produces the correct results for the given SELECT
statement.

The SELECT statement is essentlually a dubble negitive - using two LEFT
JOINS.
- The most inner nested query selects WHERE number = 1
- Then next most inner selects everything but, and
- The most outer selects every thing but everything but WHERE number = 1

The second setup (Listing C) is identicle to the first execpt that the
table "items" has an extra field and a primary key index. The goal of this
setup is to produce a cirtian query plan that I beleive is broken, where
it seems that the "Nested Loop Left Join" has forced the filter for "WHERE
number = 1" outside or (perhaps after) a join one of the more nested joins
causeing that more nested join to cancel it self out.

Well.... that's what I make of it. I really hope you are able to
reproduce this, it took me ages to find a setup that would be reproduce
what I was observing in my developement system.

See the attached output.txt for the results and the EXPLAIN statement for
the SELECT statement under each setup.

Postgres:
"PostgreSQL 8.2.3 on x86_64-unknown-linux-gnu, compiled by GCC gcc
(GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)"

Hardware:
AMD Athlon(tm) 64 X2 Dual Core Processor 4400+
2GB Mem

OS:
Kernel: Linux version 2.6.18-4-amd64 (Debian 2.6.18.dfsg.1-12)
OS: GNU/Linux Debian "Etch" - AMD64

Kind regards,
Adam Terrey

Data Systems - Marketing Strategy and Planning
University Relations Directorate
Australian Catholic University Limited (ABN 15 050 192 660)

42 Edward Street, North Sydney NSW 2060
PO Box 968 North Sydney NSW 2059
Telephone 02 9739 2517
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Mobile 0421 430 824
Email: adam(dot)terrey(at)acu(dot)edu(dot)au
http://www.acu.edu.au

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