Re: [HACKERS] CLUSTER command progress monitor

On Thu, Sep 05, 2019 at 03:17:51PM -0400, Robert Haas wrote:
> Oops.  Yeah, that's bogus (as are some of the other things you
> mention).  I think we're going to have to fix this by passing down
> some flags to these functions to tell them what kind of progress
> updates to do (or to do none).  Or else pass down a callback function
> and a context object, but that seems like it might be overkill.

One idea I got was to pass the command ID as an extra argument of the
update routine.  I am not completely sure either if we need this level
of complication.

> Those are just weaknesses of the infrastructure.  Perhaps there is a
> better solution, but there's no intrinsic reason that we can't avoid
> them by careful coding.

Perhaps.  The current infra allows the addition of a progress report
in code paths which are isolated from other things.  For CLUSTER, most
things are fine as long as the progress is updated in cluster_rel(),
the rest is too internal.

> Well, it might be OK to do that if we're clear that this is the index
> progress-reporting view and the command is CLUSTER but it happens to
> be building an index now so we're showing it here.  But I don't see
> how it would work anyway: you can't reported cascading progress
> reports in shared memory because you've only got a fixed amount of
> space.

I don't see exactly why we could not switch to a fixed number of
slots, say 8, with one code path to start a progress which adds an
extra report on the stack, one to remove one entry from the stack, and
a new one to reset the whole thing for a backend.  This would not need
much restructuration of course.

Finally comes the question of what do we do for v12?  I am adding in
CC Peter, Alvaro being already present, who have been involved in the
commits with CREATE INDEX and REINDEX.  It would be sad to revert a
this feature, but well I'd rather do that now than regret later
releasing the feature as it is currently shaped..  Let's see what the
others think.
--
Michael