On Tue, 2008-10-28 at 14:28 +0200, Heikki Linnakangas wrote: > Hannu Krosing wrote: > > On Tue, 2008-10-28 at 10:10 +0000, Simon Riggs wrote: > >> On Tue, 2008-10-28 at 11:45 +0200, Hannu Krosing wrote: > >>> On Tue, 2008-10-28 at 08:49 +0000, Simon Riggs wrote: > >>>> Looking at a VACUUM's WAL records makes me think twice about the way we > >>>> issue a VACUUM. > >>>> > >>>> 1. First we scan the heap, issuing a HEAP2 clean record for every block > >>>> that needs cleaning. > >>> IIRC the first heap pass just collects info and does nothing else. > >>> Is this just an empty/do-nothing WAL record ? > >> 8.3 changed that; it used to work that way. I guess I never looked at > >> the amount of WAL being generated. > > > > I can't see how it is safe to do anything more than just lookups on > > first pass. > > What's done in the first pass is the same HOT pruning that is done > opportunistically on other page accesses as well. IIRC it's required for > correctness, though I can't remember what exactly the issue was.
Are you sure it is a correctness thing ? Maybe HOT pruning just happened to be in a path used by vacuum to read pages. > I don't think the extra WAL volume is a problem; Probably not ( unless you need to ship your WAL records via a very expensive network connection ). If it is a simple performance problem, then it can probably be fixed by just running VACUUM slower. > VACUUM doesn't generate > much WAL, anyway. As for the extra data page writes it causes; yeah, > that might cause some I/O that could be avoided, but remember that the > first pass often dirties buffers anyway to set hint bits. Still, can't we special-case HOT pruning and hint-bit change WAL-logging for first the pass of vacuum ? They both seem redundant in case of VACUUM. --------------- Hannu -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers