Re: Wrong usage of RelationNeedsWAL

On Thu, Jan 28, 2021 at 12:06:27PM +0900, Kyotaro Horiguchi wrote:
> At Wed, 27 Jan 2021 02:48:48 -0800, Noah Misch <noah(at)leadboat(dot)com> wrote in
> > On Thu, Jan 21, 2021 at 01:23:36AM -0800, Noah Misch wrote:
> > > On Thu, Jan 21, 2021 at 06:02:11PM +0900, Kyotaro Horiguchi wrote:
> > > > Perhaps I'm missing something, but the patch doesn't pass the v5-0001
> > > > test with wal_level=minimal?
> > >
> > > Correct. The case we must avoid is letting an old snapshot read an
> > > early-pruned page without error. v5-0001 expects "ERROR: snapshot too old".
> > > The patch suspends early pruning, so that error is not applicable.

> I studied the sto feature further and concluded that the checker side
> is fine that it always follow the chages of page-LSN.
>
> So what we can do for the issue is setting seemingly correct page LSN
> at pruning or refrain from early-pruning while we are skipping
> WAL. The reason I took the former is I thought that the latter might
> be a problem since early-pruning would be postponed by a long-running
> wal-skipping transaction.

Yes, that's an accurate summary.

> So the patch looks fine to me. The commit message mekes sense.
>
> However, is it ok that the existing tests (modules/snapshot_too_old)
> fails when wal_level=minimal?

That would not be okay, but I'm not seeing that. How did your setup differ
from the following?

[nm(at)rfd 6:1 2021-01-27T23:06:33 postgresql 0]$ cat /nmscratch/minimal.conf
log_statement = all
wal_level = minimal
max_wal_senders = 0
log_line_prefix = '%m [%p %l %x] %q%a '
[nm(at)rfd 6:1 2021-01-27T23:06:38 postgresql 0]$ make -C src/test/modules/snapshot_too_old check TEMP_CONFIG=/nmscratch/minimal.conf
============== creating temporary instance ==============
============== initializing database system ==============
============== starting postmaster ==============
running on port 58080 with PID 2603099
============== creating database "isolation_regression" ==============
CREATE DATABASE
ALTER DATABASE
============== running regression test queries ==============
test sto_using_cursor ... ok 30168 ms
test sto_using_select ... ok 24197 ms
test sto_using_hash_index ... ok 6089 ms
============== shutting down postmaster ==============
============== removing temporary instance ==============

=====================
All 3 tests passed.
=====================