FAQ

On 24 May 2013 20:26, Andres Freund wrote:
On 2013-05-24 19:09:57 +0100, Simon Riggs wrote:
On 24 May 2013 18:40, Andres Freund wrote:

That pattern looks dangerous. Setting the lsn of the heap page will
prevent the next action from doing a FPI even if it would be required.
Can you be more specific about the danger you see?
CHECKPOINT at lsn 0/10;
vacuum starts
vacuum finds page which is all visible
vacuum sets all_visible
PageSetAllVisible(page);
MarkBufferDirty(buf);
visibilitymap_set(onerel, blkno, buf, InvalidXLogRecPtr,
vmbuffer, visibility_cutoff_xid);
recptr = log_heap_visible(rel->rd_node, heapBuf, vmBuf,
cutoff_xid);
if (DataChecksumsEnabled())
PageSetLSN(heapPage, recptr);

So at this point the *heap* page will have the lsn of the
xl_heap_visible record. Which I thought to be rather dangerous because I
somewow missed the fact that log_heap_visible does:
if (DataChecksumsEnabled())
{
rdata[1].next = &(rdata[2]);

rdata[2].data = NULL;
rdata[2].len = 0;
rdata[2].buffer = heap_buffer;
rdata[2].buffer_std = true;
rdata[2].next = NULL;
}

So. Forget what I said, I just was confused.
I think its perfectly understandable. Robert, Jeff and I discussed
that for a while before we passed it. I'm still not happy with it, and
think its a pretty confusing section of code with multiple paths
through it, but I just can't see a better way.

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

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grouppgsql-hackers @
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postedMay 24, '13 at 5:40p
activeMay 29, '13 at 4:54p
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