Re: HOT updates & REDIRECT line pointers

On Wed, Mar 21, 2012 at 8:13 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I wrote:
>> Robert Haas <robertmhaas@gmail.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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company