Re: [HACKERS] WAL logging problem in 9.4.3?

On Wed, Jul 04, 2018 at 07:55:53AM -0400, Andrew Dunstan wrote:
> Many thanks for working on this.

No problem. Thanks for the lookup.

> +1 for these changes, even though the TRUNCATE fix looks perverse. If
> anyone wants to propose further optimizations in this area this would
> at least give us a startpoint which is correct.

Yes, that's exactly what I am coming at. The optimizations which are
currently broken just cannot and should not be used. If anybody wishes
to improve the current set of optimizations in place for wal_level =
minimal, let's also consider the other patch. Based on the tests I sent
in the previous patch, I have compiled five scenarios by the way:
1) BEGIN -> CREATE TABLE -> TRUNCATE -> COMMIT.
With wal_level = minimal, this fails hard with "could not read block 0
blah" when trying to read the data after commit..
2) BEGIN -> CREATE -> INSERT -> TRUNCATE -> INSERT -> COMMIT, and this
one reports an empty table, without failing, but there should be tuples
from the INSERT.
3) BEGIN -> CREATE -> INSERT -> TRUNCATE -> COPY -> COMMIT, which also
reports an empty table while there should be tuples from the COPY.
4) BEGIN -> CREATE -> INSERT -> TRUNCATE -> INSERT -> COPY -> INSERT ->
COMMIT, which fails at WAL replay with a PANIC: invalid max offset
number.
5) BEGIN -> CREATE -> INSERT -> COPY -> COMMIT, which sees only the
tuple inserted, causing an incorrect number of tuples. If you reverse
the COPY and INSERT, then this is able to pass.

This stuff really generates a good number of different failures. There
have been so many people participating on this thread that discussing
more this approach would be surely a good step forward, and this
summarizes quite nicely the set of failures discussed until now here. I
would be happy to push forward with this patch to close all the holes
mentioned.
--
Michael