Re: display offset along with block number in vacuum errors

Thanks Justin.

On Sat, 1 Aug 2020 at 11:47, Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
>
> On Fri, Jul 31, 2020 at 04:55:14PM -0500, Justin Pryzby wrote:
> > Bcc:
> > Subject: Re: display offset along with block number in vacuum errors
> > Reply-To:
> > In-Reply-To: <CAKYtNApLJjAaRw0UEBBY6G1o0LRZKS7rA5n46BFh+NfwSOycdg(at)mail(dot)gmail(dot)com>
>
> whoops
>
> > 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.
>
> This makes me question whether offset numbers are ever useful during
> VACUUM_HEAP, since the real work is done a page at a time (not tuple) or by
> internal functions that don't update vacrelstats->offno. Note that my initial
> problem report that led to the errcontext implementation was an ERROR in heap
> *scan* (not vacuum). So an offset number at that point would've been
> sufficient.
> https://www.postgresql.org/message-id/20190808012436.GG11185@telsasoft.com
>
> I mentioned that lazy_check_needs_freeze() should save and restore the errinfo,
> so an error in heap_page_prune() (for example) doesn't get the wrong offset
> associated with it.
>
> So see the attached changes on top of your v2 patch.

Actually I was waiting for review comments from committer and other
people also and was planning to send a patch after that. I already
fixed your comments in my offline patch and was waiting for more
comments. Anyway, thanks for delta patch.

Here, attaching v3 patch for review.

>
> postgres=# DROP TABLE tt; CREATE TABLE tt(a int) WITH (fillfactor=90); INSERT INTO tt SELECT generate_series(1,99999); VACUUM tt; UPDATE tt SET a=1 WHERE ctid='(345,10)'; UPDATE pg_class SET relfrozenxid=(relfrozenxid::text::int + (1<<30))::int::text::xid WHERE oid='tt'::regclass; VACUUM tt;
> ERROR: found xmin 1961 from before relfrozenxid 1073743785
> CONTEXT: while scanning block 345 of relation "public.tt", item offset 205
>
> Hmm.. is it confusing that the block number is 0-indexed but the offset is
> 1-indexed ?
>
> --
> Justin

--
Thanks and Regards
Mahendra Singh Thalor
EnterpriseDB: http://www.enterprisedb.c