Re: foreign key locks

On 2013-01-18 15:37:47 -0500, Tom Lane wrote:
> Alvaro Herrera <alvherre(at)2ndquadrant(dot)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/
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