Re: Patch: Write Amplification Reduction Method (WARM)

On Fri, Mar 31, 2017 at 7:53 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> So Andres says defer this, but Robert says "more review", which is
> more than just deferral.
>
> We have some risky things in this release such as Hash Indexes,
> function changes. I perfectly understand that perception of risk is
> affected significantly by whether you wrote something or not. Andres
> and Robert did not write it and so they see problems.

While that's probably true, I don't think that's the only thing going on here:

1. Hash indexes were reviewed and reworked repeatedly until nobody
could find any more problems, including people like Jesper Pederson
who do not work for EDB and who did extensive testing. Similarly with
the expression evaluation stuff, which got some review from Heikki and
even more from Tom. Now, several people who do not work for
2ndQuadrant have recently started looking at WARM and many of those
reviews have found problems and regressions. If we're to hold things
to the same standard, those things should be looked into and fixed
before there is any talk of committing anything. My concern is that
there seems to be (even with the patches already committed) a desire
to minimize the importance of the problems that have been found --
which I think is probably because fixing them would take time, and we
don't have much time left in this release cycle. We should regard the
time between feature freeze and release as a time to fix the things
that good review missed, not as a substitute for fixing things that
should have (or actually were) found during review prior to commit.

2. WARM is a non-optional feature which touches the on-disk format.
There is nothing more dangerous than that. If hash indexes have bugs,
people can avoid those bugs by not using them; there are good reasons
to suppose that hash indexes have very few existing users. The
expression evaluation changes, IMHO, are much more dangerous because
everyone will be exposed to them, but they will not likely corrupt
your data because they don't touch the on-disk format. WARM is even a
little more dangerous than that; everyone is exposed to those bugs,
and in the worst case they could eat your data.

I agree that WARM could be a pretty great feature, but I think you're
underestimating the negative effects that could result from committing
it too soon.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company