Heikki Linnakangas wrote: > 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.
The central idea I now understand is that pruning only shrinks HOT chains. It does not allow reuse of space. Only defragmentation does that, and that is triggered when the page is almost full. -- Bruce Momjian <[EMAIL PROTECTED]> http://momjian.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
