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 :-)
Regards
From: Tom Lane <tgl@xxxxxxxxxxxxx>
To: salah jubeh <s_jubeh@xxxxxxxxx>
Cc: Volodymyr Kostyrko <c.kworr@xxxxxxxxx>; pgsql <pgsql-general@xxxxxxxxxxxxxx>
Sent: Thursday, January 26, 2012 5:47 PM
Subject: Re: [GENERAL] How to push predicate down
salah jubeh <s_jubeh@xxxxxxxxx> 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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