Date: Fri, 31 Jul 2020 16:55:14 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Mahendra Singh Thalor <mahi6run@gmail.com>
Cc: pgsql-hackers@postgresql.org,
	Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
Subject: Re: display offset along with block number in vacuum errors
Message-ID: <20200731215514.GC20393@telsasoft.com>
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Subject: Re: display offset along with block number in vacuum errors
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On Wed, Jul 29, 2020 at 12:35:17AM +0530, Mahendra Singh Thalor wrote:
> > Here:
> >
> > @@ -1924,14 +1932,22 @@ lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
> >                 BlockNumber tblk;
> >                 OffsetNumber toff;
> >                 ItemId          itemid;
> > +               LVSavedErrInfo loc_saved_err_info;
> >
> >                 tblk = ItemPointerGetBlockNumber(&dead_tuples->itemptrs[tupindex]);
> >                 if (tblk != blkno)
> >                         break;                          /* past end of tuples for this block */
> >                 toff = ItemPointerGetOffsetNumber(&dead_tuples->itemptrs[tupindex]);
> > +
> > +               /* Update error traceback information */
> > +               update_vacuum_error_info(vacrelstats, &loc_saved_err_info, VACUUM_ERRCB_PHASE_VACUUM_HEAP,
> > +                                                                blkno, toff);
> >                 itemid = PageGetItemId(page, toff);
> >                 ItemIdSetUnused(itemid);
> >                 unused[uncnt++] = toff;
> > +
> > +               /* Revert to the previous phase information for error traceback */
> > +               restore_vacuum_error_info(vacrelstats, &loc_saved_err_info);
> >         }
> >
> > I'm not sure why you use restore_vacuum_error_info() at all.  It's already
> > called at the end of lazy_vacuum_page() (and others) to allow functions to
> > clean up after their own state changes, rather than requiring callers to do it.
> > I don't think you should use it in a loop, nor introduce another
> > LVSavedErrInfo.
> >
> > Since phase and blkno are already set, I think you should just set
> > vacrelstats->offnum = toff, rather than calling update_vacuum_error_info().
> > Similar to whats done in lazy_vacuum_heap():
> >
> >                 tblk = ItemPointerGetBlockNumber(&vacrelstats->dead_tuples->itemptrs[tupindex]);
> >                 vacrelstats->blkno = tblk;
> 
> Fixed.

I rearead this thread and I think the earlier suggestion from Masahiko was
right.  The loop around dead_tuples only does ItemIdSetUnused() which updates
the page, which has already been read from disk.  On my suggestion, your v2
patch sets offnum directly, but now I think it's not useful to set at all,
since the whole page is manipulated by PageRepairFragmentation() and
log_heap_clean().  An error there would misleadingly say "..at offset number
MM", but would always show the page's last offset, and not the offset where an
error occured.

I added this at:
https://commitfest.postgresql.org/29/2662/

If anyone is considering this patch for v13, I guess it should be completed by
next week.

-- 
Justin