RE: Disable WAL logging to speed up data loading

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:

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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.
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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.

Regards
Takayuki Tsunakawa