RE: Disable WAL logging to speed up data loading

Hi, Horiguchi-San and Fujii-San.

Thank you so much both of you.
> > the table needs to be rewriitten. One idea for that is to improve that
> > command so that it skips the table rewrite if wal_level=minimal.
> > Of course, also you can change wal_level after marking the table as
> > unlogged.
>
> tablecmd.c:
The idea is really interesting.
I didn't come up with getting rid of the whole copy of
the ALTER TABLE UNLOGGED/LOGGED commands
only when wal_level='minimal'.

> > * There are two reasons for requiring a rewrite when changing
> > * persistence: on one hand, we need to ensure that the buffers
> > * belonging to each of the two relations are marked with or without
> > * BM_PERMANENT properly. On the other hand, since rewriting creates
> > * and assigns a new relfilenode, we automatically create or drop an
> > * init fork for the relation as appropriate.
Thanks for sharing concrete comments in the source code.

> According to this comment, perhaps we can do that at least for
> wal_level=minimal.
When I compare the 2 ideas,
one of the benefits of this ALTER TABLE 's improvement
is that we can't avoid the downtime
while that of wal_level='none' provides an easy and faster
major version up via output file of pg_dumpall.
Both ideas have good points.
However, actually to modify ALTER TABLE's copy
looks far more difficult than wal_level='none' and
beyond my current ability.
So, I'd like to go forward with the direction of wal_level='none'.
Did you have strong objections for this direction ?

Regards,
Takamichi Osumi