Florian Pflug wrote:
> Heikki Linnakangas wrote:
>> Tom Lane wrote:
>>> Compared to what it currently takes to check the same tuple (a separate
>>> index entry fetch and traversal to the heap page), this is already an
>>> enormous performance improvement.
>>
>> Though keep in mind that we kill index tuples as soon as they're deemed
>> to be dead. Nevertheless, I'm not very worried about the cost of
>> following the chain either. But that's something we can quite easily
>> measure if we want to.
>
> I'm confused now. I though that pruning would be enough to shorten
> HOT-Chains -
> because the root line pointer afterwards points directly to the first live
> tuple. But we can *prune* (without actually defragmenting) without holding
> a VACUUM-strength lock, right? Or did I get that wrong?
Yes, that's right. You don't seem to be confused at all.
Tom argued that following the tuple chain is cheap enough, and might
even be cheaper than what we have now, that we don't need to prune just
for the purpose of keeping the chains short. To which I pointed out that
currently, without HOT, we mark index tuples pointing to dead tuples as
killed to avoid following them in the future, so HOT without pruning is
not cheaper than what we have now.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com