On 22 July 2015 at 13:00, Robert Haas <robertmh...@gmail.com> wrote: > On Wed, Jul 22, 2015 at 3:02 AM, Simon Riggs <si...@2ndquadrant.com> > wrote: > > For me, the user workflow looks like these.... > > > > Worried: "Task X is taking ages? When is it expected to finish?" > > Ops: 13:50 > > <sometime later, about 14:00> > > Worried: "Task X is still running? But I thought its ETA was 13:50?" > > Ops: Now says 14:30 > > Worried: "Is it stuck, or is it making progress?" > > Ops: Looks like its making progress > > Worried: "Can we have a look at it and find out what its doing?" > > How does Ops know that it is making progress? Just because the > completion percentage is changing? >
You could, but that is not the way I suggested. We need * Some measure of actual progress (the definition of which may vary from action to action, e.g. blocks scanned) * Some estimate of the total work required * An estimate of the estimated time of completion - I liked your view that this prediction may be costly to request > > In terms of VACUUM specifically: VACUUM should be able to assess > beforehand > > whether it will scan the indexes, or it can just assume that it will > need to > > scan the indexes. Perhaps VACUUM can pre-scan the VM to decide how big a > > task it has before it starts. > > Well, we can assume that it will scan the indexes exactly once, but > the actual number may be more or less; and the cost of rescanning the > heap in phase 2 is also hard to estimate. > > Maybe I'm worrying over nothing, but I have a feeling that if we try > to do what you're proposing here, we're gonna end up with this: > > https://xkcd.com/612/ > > Most of the progress estimators I have seen over the ~30 years that > I've been playing with computers have been unreliable, and many of > those have been unreliable to the point of being annoying. I think > that's likely to happen with what you are proposing too, though of > course like all predictions of the future it could turn out to be > wrong. Almost like an Optimizer then. Important, often annoyingly wrong, needs more work. I'm not proposing this feature, I'm merely asking for it to be defined in a way that makes it work for more than just VACUUM. Once we have a way of reporting useful information, other processes can be made to follow that mechanism, like REINDEX, ALTER TABLE etc.. I believe those things are important, even if we never get such information for user queries. But I hope we do. I won't get in the way of your search for detailed information in more complex forms. Both things are needed. -- Simon Riggs http://www.2ndQuadrant.com/ <http://www.2ndquadrant.com/> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
