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The optimizer is too smart for me - How can I trick it?

From: "Peter Vanderborght" <peter(dot)vanderborght(at)taatu(dot)com>
To: <pgsql-general(at)postgresql(dot)org>
Subject: The optimizer is too smart for me - How can I trick it?
Date: 2008-05-30 08:49:21
Message-ID: 003d01c8c232$0e3692b0$88c8a8c0@TAATU.local (view raw or flat)
Thread:
Lists: pgsql-general
Hi,
 
I've implemented Depesz's running total function
(http://www.depesz.com/index.php/2007/08/17/rownum-anyone-cumulative-sum-in-
one-query/) in my DB, which works great.
Now what I want to do is get the running total for a certain statement and
then do a subselect on that result so to get a non-zero start on a function.
 
Example:
 
 select day, registrations, runningsum(cast('myregistrations' as text),
cast(registrations as int4)) as rtotal
 from
 (
    select cast(to_char(dateinscr, 'YYYY-MM-DD') as timestamp) as day,
count(*) as registrations
    from public.people
    where supplierid = 609
       and dateinscr < date_trunc('day', now())
    group by day
    order by day
 ) subQ

This gives me correct output like this:
 
         day         | registrations | rtotal 
---------------------+---------------+--------
 2008-04-14 00:00:00 |             1 |      1
 2008-04-17 00:00:00 |            11 |     12
 2008-04-18 00:00:00 |            24 |     36
 2008-04-19 00:00:00 |            14 |     50
 2008-04-20 00:00:00 |            13 |     63
 2008-04-21 00:00:00 |             6 |     69
 2008-04-22 00:00:00 |             2 |     71
 2008-04-23 00:00:00 |            12 |     83
 2008-04-24 00:00:00 |             5 |     88
 2008-04-25 00:00:00 |            13 |    101
 2008-04-26 00:00:00 |            11 |    112

Then I want to subselect on this and do:

select *
from
(
 select day, registrations, runningsum(cast('myregistrations' as text),
cast(registrations as int4)) as rtotal
 from
 (
    select cast(to_char(dateinscr, 'YYYY-MM-DD') as timestamp) as day,
count(*) as registrations
    from public.people
    where supplierid = 609
       and dateinscr < date_trunc('day', now())
    group by day
    order by day
 ) subQ
) subq2
Where day > '2008-04-24'

And I would expect to get
         day         | registrations | rtotal 
---------------------+---------------+--------
 2008-04-25 00:00:00 |            13 |    101
 2008-04-26 00:00:00 |            11 |    112

Instead, the optimizer sees what I'm trying to do, moves the where clause
inside the subquery and my output becomes

         day         | registrations | rtotal 
---------------------+---------------+--------
 2008-04-25 00:00:00 |            13 |     13
 2008-04-26 00:00:00 |            11 |     24

What can I do to tell the optimizer to keep its hands off my query or at
least get it to not optimize?


Thanks,
Peter 


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