On Thu, Aug 4, 2016 at 10:58:33PM +0530, Pavan Deolasee wrote:
> 2. I also think that most often the tuple that will be updated will have its
> t_ctid pointing to itself. This sounds more invasive, but can we somehow
Agreed! I think it has to. Great idea.
> utilise the t_ctid field to store the OffsetNumber of the root line pointer?
> The root line pointer only exists when the tuple under consideration has
> HEAP_ONLY_TUPLE flag set and we know for such tuples, BlockNumber in t_ctid
> must be same as current block (unless HEAP_UPDATED is also set and the updating
> transaction eventually aborted).
Oh, aborts, so the tuple we are looking at is the head of the chain and
points to a tuple that was aborted. So we would still need a page-scan
to handle cases where the t_ctid was over-written by an aborted tuple,
but as you said it would be rare.
So, if the tuple has HEAP_ONLY_TUPLE set, we know the block number
ip_blkid equals our block number, so any ip_blkid value that doesn't
equal our block number is a special flag that indicates that ip_posid
points to the head of our HOT chain. I guess we could just set it to
(our block number + 1), with rollover possible and desired.
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
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