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Re: HOT updates & REDIRECT line pointers

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: HOT updates & REDIRECT line pointers
Date: 2012-03-22 00:41:33
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Wed, Mar 21, 2012 at 8:13 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> Specifically, I'm wondering if we couldn't get away with rearranging
>>> things so that the root line pointer (which has index entries) points
>>> to the actual tuple, and the other line pointer (which can't have any
>>> index entries) gets marked UNUSED.
>> This would amount to changing the TID of the live row.
> Another issue, quite independent from race conditions against other
> observers of the row, is what if the tuple is part of an update chain?
> You have no way to find the predecessor row version and update its
> t_ctid forward link.

I don't see why I need to.  The predecessor, if any, points to the
root of the HOT chain; and that's exactly the TID that I'm proposing
to keep around.  The heap-only tuple's TID gets canned, but nobody can
be pointing to that from outside the block, IIUC.

Robert Haas
The Enterprise PostgreSQL Company

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