On Fri, Feb 24, 2017 at 3:42 PM, Robert Haas <robertmh...@gmail.com> wrote:
> > > Wow, OK. In my view, that makes the chain conversion code pretty much > essential, because if you had WARM without chain conversion then the > visibility map gets more or less irrevocably less effective over time, > which sounds terrible. Yes. I decided to complete chain conversion patch when I realised that IOS will otherwise become completely useful if large percentage of rows are updated just once. So I agree. It's not an optional patch and should get in with the main WARM patch. > But it sounds to me like even with the chain > conversion, it might take multiple vacuum passes before all visibility > map bits are set, which isn't such a great property (thus e.g. > fdf9e21196a6f58c6021c967dc5776a16190f295). > > The chain conversion algorithm first converts the chains during vacuum and then checks if the page can be set all-visible. So I'm not sure why it would take multiple vacuums before a page is set all-visible. The commit you quote was written to ensure that we make another attempt to set the page all-visible after al dead tuples are removed from the page. Similarly, we will convert all WARM chains to HOT chains and then check for all-visibility of the page. Thanks, Pavan -- Pavan Deolasee http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
