From: Tom Lane <tgl@sss.pgh.pa.us>
To: Stephan Szabo <sszabo@megazone.bigpanda.com>
Cc: Russell Simpkins <russellsimpkins@hotmail.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: after delete trigger behavior
Date: Wed, 22 Jun 2005 15:46:41 -0400
Message-ID: <20666.1119469601@sss.pgh.pa.us> (raw)
In-Reply-To: <20050622122313.F42989@megazone.bigpanda.com>
References: <BAY103-F108CECCE4CF4F95511684B5EB0@phx.gbl>
<20050622092239.T32802@megazone.bigpanda.com>
<20364.1119467550@sss.pgh.pa.us>
<20050622122313.F42989@megazone.bigpanda.com>
Stephan Szabo <sszabo@megazone.bigpanda.com> writes:
> Is there anything we have right now that will handle this kind of thing
> without requiring either updating all the counts after a deletion in a
> statement trigger or once per row updating all the counts for records with
> the same "a" (doing something like make a sequence and using it in a
> subselect matching keys)?
The best thing I can think of is your first idea, ie, renumbering all
the rows in a statement-level AFTER DELETE trigger. Something like
(untested)
DECLARE
rec record;
n integer := 1;
BEGIN
FOR rec IN
SELECT * FROM table
WHERE <<grouping cols = rec's grouping cols>>
ORDER BY sort_order
LOOP
IF rec.sort_order != n THEN
UPDATE table SET sort_order = n
WHERE <<primary key = rec's primary key>>;
END IF;
n := n + 1;
END LOOP;
END;
Ugly as this is, it's at least linear in the number of rows to be
changed; the originally proposed trigger was O(N^2) in the number of
rows affected, and would surely be intolerably slow for multiple deletes
in a reasonably sized table. Given an index on the grouping columns
plus sort_order, it could even be reasonably fast (don't forget to make
the ORDER BY match the index).
regards, tom lane
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, sszabo@megazone.bigpanda.com, russellsimpkins@hotmail.com
Subject: Re: after delete trigger behavior
In-Reply-To: <20666.1119469601@sss.pgh.pa.us>
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