Date: Wed, 25 Mar 2020 23:41:15 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	Andres Freund <andres@anarazel.de>,
	Michael Paquier <michael@paquier.xyz>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: error context for vacuum to include block number
Message-ID: <20200326044115.GB28385@telsasoft.com>
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On Thu, Mar 26, 2020 at 09:50:53AM +0530, Amit Kapila wrote:
> > > after count_nondeletable_pages, and then revert it back to
> > > VACUUM_ERRCB_PHASE_SCAN_HEAP phase and the number of blocks of
> > > relation before truncation, after RelationTruncate(). It can be
> > > repeated until no more truncating can be done. Why do we need to
> > > revert back to the scan heap phase? If we can use
> > > vacrelstats->nonempty_pages in the error context message as the
> > > remaining blocks after truncation I think we can update callback
> > > arguments once at the beginning of lazy_truncate_heap() and don't
> > > revert to the previous phase, and pop the error context after exiting.
> >
> > Perhaps.  We need to "revert back" for the vacuum phases, which can be called
> > multiple times, but we don't need to do that here.
> 
> Yeah, but I think it would be better if are consistent because we have
> no control what the caller of the function intends to do after
> finishing the current phase.  I think we can add some comments where
> we set up the context (in heap_vacuum_rel) like below so that the idea
> is more clear.
> 
> "The idea is to set up an error context callback to display additional
> information with any error during vacuum.  During different phases of
> vacuum (heap scan, heap vacuum, index vacuum, index clean up, heap
> truncate), we update the error context callback to display appropriate
> information.
> 
> Note that different phases of vacuum overlap with each other, so once
> a particular phase is over, we need to revert back to the old phase to
> keep the phase information up-to-date."

Seems fine.  Rather than saying "different phases" I, would say:
"The index vacuum and heap vacuum phases may be called multiple times in the
middle of the heap scan phase."

But actually I think the concern is not that we unnecessarily "Revert back to
the old phase" but that we do it in a *loop*.  Which I agree doesn't make
sense, to go back and forth between "scanning heap" and "truncating".  So I
think we should either remove the "revert back", or otherwise put it
after/outside the "while" loop, and change the "return" paths to use "break".

-- 
Justin