From: Michael Glaesemann <grzm@seespotcode.net>
To: Bruno Wolff III <bruno@wolff.to>
Cc: Tomas Vondra <tv@fuzzy.cz>
Cc: pgsql-sql@postgresql.org
Subject: Re: implementing (something like) UNIQUE constraint using PL/pgSQL
Date: Sat, 27 Jan 2007 20:58:24 +0900
Message-ID: <51AAD09D-F90D-480F-A93E-0C57246415D7@seespotcode.net> (raw)
In-Reply-To: <20070127075505.GA8035@wolff.to>
References: <45B9CCC6.4000109@fuzzy.cz>
<20070127075505.GA8035@wolff.to>
On Jan 27, 2007, at 16:55 , Bruno Wolff III wrote:
> On Fri, Jan 26, 2007 at 10:41:26 +0100,
> Tomas Vondra <tv@fuzzy.cz> wrote:
>>
>> in our application we need to implement a constraint that enforces
>> 'at
>> most N rows with this value', that is we have a table with 'flag'
>> column
>> and for each value there should be at most 10 rows (for example, the
>> exact number does not matter).
>
> Another approach is to add a instance number column and constrain that
> value to be between 1 and 10. And make value and instance number
> unique.
> You'll need to do a bit more work when inserting new rows than normal
> (to find a free instance number). This should be very robust against
> getting in a bad state.
> If you go with enforcing the condition with a trigger you need to
> be careful
> about simultaneous inserts and visibility.
This may have already been mentioned upthread, but this sounds like a
special case of the SQL queues I saw on Greg Sabino Mullane's blog:
http://people.planetpostgresql.org/greg/index.php?/archives/89-
Implementing-a-queue-in-SQL-Postgres-version.html
Perhaps you could tweak that to serve your needs.
Michael Glaesemann
grzm seespotcode net
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To: pgsql-sql@postgresql.org
Cc: grzm@seespotcode.net, bruno@wolff.to, tv@fuzzy.cz
Subject: Re: implementing (something like) UNIQUE constraint using PL/pgSQL
In-Reply-To: <51AAD09D-F90D-480F-A93E-0C57246415D7@seespotcode.net>
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