From: Stefan Weiss <spaceman@foo.at>
To: PFC <lists@boutiquenumerique.com>
Cc: Bruno Wolff III <bruno@wolff.to>
Cc: pgsql-sql@postgresql.org
Subject: Re: Links between rows in a table
Date: Sun, 06 Mar 2005 21:07:49 +0100
Message-ID: <422B6315.1050904@foo.at> (raw)
In-Reply-To: <opsm8dm0z7th1vuj@musicbox>
References: <422A8A26.1070704@foo.at>
<20050306174201.GA19019@wolff.to>
<422B4B9A.8020606@foo.at>
<opsm8dm0z7th1vuj@musicbox>
On 2005-03-06 20:26, PFC wrote:
> Because your relation is symmetric, you should not name them "user" and
> "friend".
A good point, thank you.
> user_id_1 < user_id_2 means :
> - a user can't be his own friend
> - only one row per friend
> - when you want to know if A is friend of B, no need to make two selects,
> just select where user_id_1 = min(user_id_A, user_id_B) AND user_id_2 =
> max(user_id_A, user_id_B)
This is what we were planning to do on the application side, but a CHECK
constraint is even better. It will be used and enforced by those DB
engines that understand it, and ignored by the one engine that doesn't.
> To get the list of friends for a user, you still need the union, but that
> is no real problem. Making two queries will be marginally slower than one
> query on a bigger table, but youu save precious cache space, so in the end
> it could be faster.
Thank you for your insight. We will rename the columns, add the CHECK
and go ahead with this setup.
regards,
stefan weiss
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To: pgsql-sql@postgresql.org
Cc: spaceman@foo.at, lists@boutiquenumerique.com, bruno@wolff.to
Subject: Re: Links between rows in a table
In-Reply-To: <422B6315.1050904@foo.at>
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