Re: pg_config wrongly marked as not parallel safe?

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: laurenz(dot)albe(at)cybertec(dot)at, andres(at)anarazel(dot)de, mail(at)joeconway(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: pg_config wrongly marked as not parallel safe?
Date: 2018-11-29 02:17:17
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* Kyotaro HORIGUCHI (horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp) wrote:
> At Tue, 27 Nov 2018 06:33:42 +0100, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote in <8dcb0bb248e9aaef0f1ef0faa27ab583bfa5bb84(dot)camel(at)cybertec(dot)at>
> > On Mon, 2018-11-26 at 19:51 -0500, Stephen Frost wrote:
> > > If a function's results can change across minor or major versions, we
> > > shouldn't be marking it as immutable because, by definition, it's not
> > > immutable.
> > >
> > > We, today, have a baked in assumption that any function marked as
> > > immutable will remain immutable across all major versions that we allow
> > > indexes to be retained through, which is all of them since about 8.3 at
> > > this point.
> > >
> > > We absolutely need a policy that if you decide to change the results of
> > > some immutable function across a major version change, you need to
> > > consider the results on indexes and possibly write into pg_upgrade
> > > checks to try and detect any usage of that immutable function. I hope
> > > we're in agreement there.
> >
> > It's hard to make a guarantee that a function will never change in the
> > future. What if we fix some rounding or overflow problem in a floating
> > point function?
> +1.
> Actually, some geometric comparisons are performed counting
> tolerance margin, the validity of which is in doubt. Their
> behavior has been changed in recent major version and still has a
> room for improvement, and the functions are parallel-safe and
> immutable. Immutablity is mentiond mainly in the light of
> optimization in the documentation.

I really don't buy off on these arguments in the least. I also didn't
say that a function wasn't allowed to change- but that the output of an
immutable function, for a given input, shouldn't change and in the very
rare case where we absolutely had to make a change, it had better be for
a very good reason and we need to consider the impact on user indexes.

I don't agree that we can simply declare that all functional and partial
indexes have to be rebuilt between every major version upgrade, which is
the alternative. There really isn't another option- either we're
extremely careful and take immutability of functions seriously and make
sure to preserve behavior, and therefore indexes, across major versions,
or we don't and we require indexes to be rebuilt.

When it comes to the documentation, perhaps we should improve it to make
it clear that if you change the results from an immutable function then
you need to rebuild any indexes which use it.

> > If we went that far, hardly any function could be IMMUTABLE.

I don't agree.

> > I think it is best to use IMMUTABLE whenever we don't expect it to
> > change and it is a function useful for indexes, and if it happens to
> > change nonetheless, write into the release notes that certain indexes
> > have to be rebuilt after upgrade.

I don't agree with the notion of "if it happens to change" as that
sounds pretty cavalier, and only noting something in the release notes
is a good way to cause a lot of pain for users. If it's possible to do
better, which it often is (see the list of things pg_upgrade already
checks...) then we should do so. If there's reasons we can't, fine, it
happens, but that needs to be taken into account when we're considering
making such a change and its impact on users.

> > Of course, there is no problem to mark pg_config as stable, because
> > there is little chance it will be used in an index anyway.
> Agreed. We have the following description in the documentation.

I'm pretty sure we all agree that pg_config should be marked as stable
and that changing it from immutable to stable won't be an issue.

> | A common error is to label a function IMMUTABLE when its results
> | depend on a configuration parameter. For example, a function that
> | manipulates timestamps might well have results that depend on the
> | TimeZone setting. For safety, such functions should be labeled
> | STABLE instead.
> Still this isn't mentioning upgrades. We could add to this that
> something like that immutablity is not guaranteed beyond
> PostgreSQL versions, the results of immutable functions might
> change for certain reasons including bug fixes, blah, blah..

Which means we're telling every user out there that they have to rebuild
every functional or partial index they have across every major version.

I don't agree with that and I don't think we really have some pressing
need to be that free with making changes to immutable functions.

I do think it'd be good to make a mention when talking about immutable
functions that they're allowed to be used in indexes and, as such, users
need to be careful to make sure that they don't change the results from
an immutable function unless they're sure it's not used in an index or
they're prepared to rebuild all indexes where it is used.



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