Date: Thu, 6 Aug 2020 09:21:16 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Mahendra Singh Thalor <mahi6run@gmail.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: display offset along with block number in vacuum errors
Message-ID: <20200806142116.GL28072@telsasoft.com>
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On Thu, Aug 06, 2020 at 07:39:21PM +0530, Amit Kapila wrote:
> On Wed, Jul 29, 2020 at 1:09 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
> >
> > On Wed, Jul 29, 2020 at 12:35:17AM +0530, Mahendra Singh Thalor wrote:
> > > Apart from these, I fixed comments given by Sawada and Michael in the
> > > latest patch. Attaching v2 patch for review.
> >
> > Thanks.
> >
> > lazy_check_needs_freeze iterates over blocks and this patch changes it to
> > update vacrelstats.  I think it should do what
> > lazy_{vacuum/cleanup}_heap/page/index do and call update_vacuum_error_info() at
> > its beginning (even though only the offset is changed), and then
> > restore_vacuum_error_info() at its end (to "revert back" to the item number it
> > started with).
> >
> 
> I see that Mahendra has changed patch as per this suggestion but I am
> not convinced that it is a good idea to sprinkle
> update_vacuum_error_info()/restore_vacuum_error_info() at places more
> than required. I see that it might look a bit clean from the
> perspective that if tomorrow we use the function
> lazy_check_needs_freeze() for a different purpose then we don't need
> to worry about the wrong phase information. If we are worried about
> that then we should have an assert in that function to ensure that the
> current phase is VACUUM_ERRCB_PHASE_SCAN_HEAP.

The motivation was to restore the offnum, which is set to Invalid at the start
of lazy_scan_heap(), and then set valid within lazy_check_needs_freeze, but
should be restored or re-set to Invalid when returns to lazy_scan_heap().  If
you think it's important, we could just set vacrelstats->offnum = Invalid
before returning, but that's what the restore function was built for.  We do
direct assignment in 2 places to avoid a function call within a loop.

lazy_scan_heap(Relation onerel, VacuumParams *params, LVRelStats *vacrelstats,
                           Relation *Irel, int nindexes, bool aggressive)

...
        for (blkno = 0; blkno < nblocks; blkno++)
        {
...
                update_vacuum_error_info(vacrelstats, NULL, VACUUM_ERRCB_PHASE_SCAN_HEAP,
                                                                 blkno, InvalidOffsetNumber);
                if (!ConditionalLockBufferForCleanup(buf))
                {
...
                        if (!lazy_check_needs_freeze(buf, &hastup, vacrelstats))
                        {
...
                for (offnum = FirstOffsetNumber;
                         offnum <= maxoff;
                         offnum = OffsetNumberNext(offnum))


-- 
Justin