On 21 October 2015 at 13:31, Jeff Janes <jeff.ja...@gmail.com> wrote:
> On Tue, Oct 20, 2015 at 7:02 AM, Robert Haas <robertmh...@gmail.com> > wrote: > >> On Sun, Oct 18, 2015 at 5:23 PM, Jeff Janes <jeff.ja...@gmail.com> wrote: >> > I'm planning on adding a todo item to have COPY FREEZE set >> PD_ALL_VISIBLE. >> > Or is there some reason this can't be done? >> > >> > Since the whole point of COPY FREEZE is to avoid needing to rewrite the >> > entire table, it seems rather perverse that the first time the table is >> > vacuumed, it needs to rewrite the entire table. >> >> *facepalm* >> >> I don't know how hard that is to implement, but +1 for trying to >> figure out a way. >> > > > It turns out it was pretty easy to set PD_ALL_VISIBLE on the new pages, > since the code in hio that requests the relation to be extended already has > info on the tuple's intended freeze status. > > Then you just need to refrain from clearing PD_ALL_VISIBLE when that tuple > is actually written into the page. Not only because clearing would defeat > the purpose, but also because it will cause an error--apparently the > incipient page is not yet in a state where visibilitymap_clear is willing > to deal with it. > > With this patch, you get a table which has PD_ALL_VISIBLE set for all > pages, but which doesn't have a _vm file. > Patch is simple enough. All usage looks safe, so I reckon this is good. Index-only scans will visit the heap for each tuple until the first VACUUM > is done. > > The first vacuum will read the entire table, but not need to write it > anymore. And will create the _vm file. > > I think we really want to create _vm file as well as set PD_ALL_VISIBLE, > but I don't know the best way to do that. Set a flag somewhere and then > create it in bulk at the end of the transaction? Set it bit by bit as the > pages are extended and initialized? > Easy enough to do it at the end of the COPY FREEZE in one step. No need to wait until EOXact. -- Simon Riggs http://www.2ndQuadrant.com/ <http://www.2ndquadrant.com/> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
