Re: [PROPOSAL] VACUUM Progress Checker.

On Wed, Jul 22, 2015 at 3:02 AM, Simon Riggs <simon(at)2ndquadrant(dot)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?

> 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.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company