Date: Fri, 18 Jan 2013 21:46:43 +0100
From: Andres Freund <andres@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>, Erik Rijkers <er@xs4all.nl>,
	Kevin Grittner <kgrittn@mail.com>, pgsql-hackers@postgresql.org
Subject: Re: foreign key locks
Message-ID: <20130118204643.GA4359@awork2.anarazel.de>
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	<20121228204847.GD4150@alvh.no-ip.org>
	<20130110210040.GB4110@alvh.no-ip.org>
	<20130118200204.GH4063@alvh.no-ip.org>
	<28623.1358541467@sss.pgh.pa.us>
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On 2013-01-18 15:37:47 -0500, Tom Lane wrote:
> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> > The reason is that there is an (unknown to me) rule that there must be
> > some data not associated with a buffer:
> 
> > 	/*
> > 	 * NOTE: We disallow len == 0 because it provides a useful bit of extra
> > 	 * error checking in ReadRecord.  This means that all callers of
> > 	 * XLogInsert must supply at least some not-in-a-buffer data. [...]
> > 	 */
> 
> > This seems pretty strange to me.  And having the rule be spelled out
> > only in a comment within XLogInsert and not at its top, and not nearby
> > the XLogRecData struct definition either, seems pretty strange to me.
> > I wonder how come every PG hacker except me knows this.
> 
> I doubt it ever came up before.  What use is logging only the content of
> a buffer page?  Surely you'd need to know, for example, which relation
> and page number it is from.

It only got to be a length of 0 because the the data got removed due to
a logged full page write. And the backup block contains the data about
which blocks it is logging in itself.
I wonder if the check shouldn't just check write_len instead, directly
below the loop that ads backup blocks.

Greetings,

Andres Freund

-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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