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Re: How to push predicate down

From: salah jubeh <s_jubeh(at)yahoo(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Volodymyr Kostyrko <c(dot)kworr(at)gmail(dot)com>, pgsql <pgsql-general(at)postgresql(dot)org>
Subject: Re: How to push predicate down
Date: 2012-01-26 23:33:17
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-general
I think in my case, It is safe to push the predicate down.  Can someone please, examine the behavior of other databases.If it behaves like postgres, I will assume there are some cases where it can lead to wrong result set.  I tried SQL server but my windows refuses it :-)


 From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: salah jubeh <s_jubeh(at)yahoo(dot)com> 
Cc: Volodymyr Kostyrko <c(dot)kworr(at)gmail(dot)com>; pgsql <pgsql-general(at)postgresql(dot)org> 
Sent: Thursday, January 26, 2012 5:47 PM
Subject: Re: [GENERAL] How to push predicate down 
salah jubeh <s_jubeh(at)yahoo(dot)com> writes:
> Sorry,  The scenario, that I posted was not correct. I have traced it and the union was not the problem, As I said the query excusion plan is over 5000 line. I have created a scenario which similar to the scenario causes the problem I have. 

> [ query uses EXCEPT not UNION ]

Hmm.  The code explicitly won't push conditions down through an EXCEPT:

* 3. If the subquery contains EXCEPT or EXCEPT ALL set ops we cannot push
* quals into it, because that could change the results.

I remember coming to the conclusion that this is safe for
UNION/INTERSECT but not EXCEPT, but right at the moment I'm not sure why
I thought that --- it seems like a qual that suppresses specific rows
should suppress all matching copies.

            regards, tom lane

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