Hi, On 2020-03-17 21:58:53 -0400, James Coleman wrote: > On Tue, Mar 17, 2020 at 9:03 PM Andres Freund <and...@anarazel.de> wrote: > > > > Hi, > > > > On 2020-03-17 20:42:07 +0100, Laurenz Albe wrote: > > > > I think Andres was thinking this would maybe be an optimization > > > > independent of > > > > is_insert_only (?) > > > > > > I wasn't sure. > > > > I'm not sure myself - but I'm doubtful that using a 0 min age by default > > will be ok. > > > > I was trying to say (in a later email) that I think it might be a good > > compromise to opportunistically freeze if we're dirtying the page > > anyway, but not optimize WAL emission etc. That's a pretty simple > > change, and it'd address a lot of the potential performance regressions, > > while still freezing for the "first" vacuum in insert only workloads. > > If we have truly insert-only tables, then doesn't vacuuming with > freezing every tuple actually decrease total vacuum cost (perhaps > significantly) since otherwise every vacuum keeps having to scan the > heap for dead tuples on pages where we know there are none? Those > pages could conceptually be frozen and ignored, but are not frozen > because of the default behavior, correct?
Yes. > If that's all true, it seems to me that removing that part of the > patch significantly lowers its value. Well, perfect sometimes is the enemy of the good. We gotta get something in, and having some automated vacuuming for insert mostly/only tables is a huge step forward. And avoiding regressions is an important part of doing so. I outlined the steps we could take to allow for more aggressive vacuuming upthread. > If we opportunistically freeze only if we're already dirtying a page, > would that help a truly insert-only workload? Yes. > E.g., are there hint bits on the page that would need to change the > first time we vacuum a full page with no dead tuples? Yes. HEAP_XMIN_COMMITTED. > I would have assumed the answer was "no" (since if so I think it would > follow that _all_ pages need updated the first time they're > vacuumed?). That is the case. Although they might already be set when the tuples are accessed for other reasons. Greetings, Andres Freund
