To: Tony Cebzanov <tonyceb@andrew.cmu.edu>
cc: Richard Huxton <dev@archonet.com>, pgsql-sql@postgresql.org
Subject: Re: Avoiding cycles in a directed graph 
In-reply-to: <4B9FEEE8.1060404@andrew.cmu.edu> 
References: <4B9FDFDE.5030005@andrew.cmu.edu> <4B9FE752.4080709@archonet.com>
	<14113.1268771643@sss.pgh.pa.us> <4B9FEEE8.1060404@andrew.cmu.edu>
Comments: In-reply-to Tony Cebzanov <tonyceb@andrew.cmu.edu>
	message dated "Tue, 16 Mar 2010 16:49:44 -0400"
Date: Tue, 16 Mar 2010 17:09:12 -0400
Message-ID: <14550.1268773752@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Tony Cebzanov <tonyceb@andrew.cmu.edu> writes:
> I'm okay with running the big, fat WITH RECURSIVE query in my insert
> trigger if I have to -- it won't be great for performance, but I don't
> expect this to be a frequent operation, so I'll accept the performance
> hit if it works.

> Unfortunately I can't even get that working.  Here's the (not at all
> functional) trigger I've got right now, which only detects the cycle
> *after* it's been inserted, which is of no help at all.  Any way I can
> modify this to do the right thing?

Run it in an AFTER trigger?

If you don't expect this to be common, maybe you could fix the
concurrency issue by taking a table-wide lock that locks out
other writers.

			regards, tom lane