Date: Sun, 08 Jan 2006 16:13:01 +1300
From: Mark Kirkwood <markir@paradise.net.nz>
Subject: Re: Summary table trigger example race condition
In-reply-to: <43BDE812.5000103@paradise.net.nz>
To: Mark Kirkwood <markir@paradise.net.nz>
Cc: "Jim C. Nasby" <jnasby@pervasive.com>, pgsql-patches@postgresql.org,
	pgsql-docs@postgresql.org
Message-id: <43C0833D.4040607@paradise.net.nz>
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References: <20060105213645.GY43311@pervasive.com>
	<43BDC132.90007@paradise.net.nz> <20060106030415.GL43311@pervasive.com>
	<43BDE812.5000103@paradise.net.nz>

Mark Kirkwood wrote:
> Jim C. Nasby wrote:
> 
>> On Fri, Jan 06, 2006 at 02:00:34PM +1300, Mark Kirkwood wrote:
>>
>>> However, I think the actual change is not quite right - after running 
>>
>>
>>
>> DOH! It would be good if doc/src had a better mechanism for handling
>> code; one that would allow for writing the code natively (so you don't
>> have to worry about translating < into &lt; and > into &gt;) and for
>> unit testing the different pieces of code.
>>
> 
> Yes it would - I usually build the SGML -> HTML, then cut the code out 
> of a browser session to test - the pain is waiting for the docs to build.
> 
>> Anyway, updated patch attached.
>>
> 
> This one is good!
> 

After re-examining the original code, it looks like it was not actually 
vulnerable to a race condition! (it does the UPDATE, then if not found 
will do an INSERT, and handle unique violation with a repeat of the same 
UPDATE - i.e three DML statements, which are enough to handle the race 
in this case).

However Jim's change handles the race needing only two DML statements in 
a loop, which seems much more elegant! In addition it provides a nice 
example of the 'merge' style code shown in e.g 36-1.

Cheers

Mark