Re: Disable WAL logging to speed up data loading

At Fri, 8 Jan 2021 05:12:02 +0000, "tsunakawa(dot)takay(at)fujitsu(dot)com" <tsunakawa(dot)takay(at)fujitsu(dot)com> wrote in
> From: Robert Haas <robertmhaas(at)gmail(dot)com>
> > Were the issues that I mentioned regarding GIST (and maybe other AMs)
> > in the last paragraph of
> > http://postgr.es/m/CA+TgmoZEZ5RONS49C7mEpjhjndqMQtVrz_LCQUkpRW
> > dmRevDnQ(at)mail(dot)gmail(dot)com
> > addressed in some way? That seems like a pretty hard engineering
> > problem to me, and I don't see that there's been any discussion of it.
> > Those are correctness concerns separate from any wal_level tracking we
> > might want to do to avoid accidental mistakes.
>
> Thank you very much for reminding me of this. I forgot I replied as follows:
>
>
> --------------------------------------------------
> Unlogged GiST indexes use fake LSNs that are instance-wide. Unlogged temporary GiST indexes use backend-local sequence values. Other unlogged and temporary relations don't set LSNs on pages. So, I think it's enough to call GetFakeLSNForUnloggedRel() when wal_level = none as well.
> --------------------------------------------------
>
>
> But this is not correct. We have to allow (RM_GIST_ID,
> XLOG_GIST_ASSIGN_LSN) WAL records to be emitted (by tweaking the
> filter in XLogInsert()), just like those WAL records are emitted
> when wal_level = minimal and and other WAL records are not emitted.

Yeah, although LOGGED and UNLOGGED use incompatible LSNs, LOGGED
tables always uses real LSNs, maintained by that type of WAL record
while wal_level = minimal.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center