Re: [HACKERS] WAL logging problem in 9.4.3?

Thank you for looking this.

At Wed, 3 Apr 2019 10:16:02 -0400, Robert Haas <robertmhaas@gmail.com> wrote in
<CA+TgmoYEST4xYaU10gM=XXeA-oxbFh=qSfy0X4PXDCWubcgj=g@mail.gmail.com>
> On Tue, Apr 2, 2019 at 6:54 AM Kyotaro HORIGUCHI
> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
> > > By using DELETE and INSERT records to implement an UPDATE, you lose the ctid
> > > chain and infomask bits that were present before crash recovery.  If that's
> > > okay in these circumstances, please write a comment explaining why.
> >
> > Sounds reasonable. Added a comment. (Honestly I completely forgot
> > about that.. Thanks!) (0006)
> 
> If you haven't already, I think you should set up a master and a
> standby and wal_consistency_checking=all and run tests of this feature
> on the master and see if anything breaks on the master or the standby.
> I'm not sure that emitting an insert or delete record is going to
> reproduce the exact same state on the standby that exists on the
> master.

All of this patch is for wal_level = minimal. Doesn't make
changes in other cases. Updates are always replicated as
XLOG_HEAP_(HOT_)UPDATE. Crash recovery cases involving log_insert
or log_update are exercised by the TAP test.

> + * Insert log record. Using delete or insert log loses HOT chain
> + * information but that happens only when newbuf is different from
> + * buffer, where HOT cannot happen.
> 
> "HOT chain information" seems pretty vague.

Thanks. Actually I was a bit uneasy with "information". Does the
following make sense?

> * Insert log record, using delete or insert instead of update log
> * when only one of the two buffers needs WAL-logging. If this were a
> * HOT-update, redoing the WAL record would result in a broken
> * hot-chain. However, that never happens because updates complete on
> * a single page always use log_update.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center