Message-ID: <54457869.3030601@enercoop.org>
Date: Mon, 20 Oct 2014 23:02:33 +0200
From: Laurent Martelli <laurent.martelli@enercoop.org>
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To: pgsql-performance@postgresql.org
Subject: Re: IS NOT NULL and LEFT JOIN
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Le 20/10/2014 15:58, Tom Lane a écrit :
> Laurent Martelli <laurent.martelli@enercoop.org> writes:
>> Do we agree that both queries are identical ?
> No, they *aren't* identical.  Go consult any SQL reference.  Left join
> conditions don't work the way you seem to be thinking: after the join,
> the RHS column might be null, rather than equal to the LHS column.
Yes, I was wrong to assume that c.user_info=u.id because of the LEFT JOIN.

But since I only want rows where u.id IS NOT NULL, in any case I will 
also have c.user_info IS NOT NULL.

Also, having a foreign key, if c.user_info is not null, it will have a 
match in u. So in that case, either both c.user_info and c.id are null 
in the result rows, or they are equal.

Regards,
Laurent


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