On Jan 27, 2007, at 16:55 , Bruno Wolff III wrote:
On Fri, Jan 26, 2007 at 10:41:26 +0100,
Tomas Vondra <[EMAIL PROTECTED]> 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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