Re: [HACKERS] proposal: session server side variables

On 30 December 2016 at 16:46, Fabien COELHO <coelho@cri.ensmp.fr> wrote:
>
>> Pavel's personal requirements include that it be well suited for
>> static analysis of plpgsql using his plpgsql_check tool. So he wants
>> persistent definitions.
>
>
> I've been in static analysis for the last 25 years, and the logic of this
> statement fails me.

I have no opinion here, as I've not seen plpgsql_check nor do I
understand the issues Pavel perceives with having dynamic definitions
of variables.

All I'm saying is that you two are talking around in circles by
repeating different requirements to each other, and it's not going to
get anywhere unless you both change your approach. It sounds like
you're already trying to do that.

> I do not think that a feature should be designed around the current
> limitations of a particular external tool, esp. if said tool can be improved
> at a reasonable cost.

Not arguing there.

I was initially inclined to favour Pavel's proposal because it fits a
RLS use case I was somewhat interested in. But so would dynamic
variables resolved at runtime so long as they were fast.

Personally I don't much care what the result is, so long as it can
satisfy some kind of reasonable security isolation, such that role A
can set it, B can read it but not set it, and role C can do neither.
Preferably without resorting to creating SECURITY DEFINER accessors,
since they're messy and slow. Support for data typing would also be
nice too.

If it doesn't deliver security controls then IMO there's not much
advantage over (ab)using GUCs with current_setting(...).

Exploring the other areas discussed:

Personally I think MVCC, persistent variables are a totally
unnecessary idea that solves a problem we don't have. But maybe I
don't understand your use cases. I expect anything like that would
land up using a pg_catalog relation as a k/v-like store with different
columns for different types or something of the like, which is really
something the user can do well enough for themselves. I don't see the
point at all.

Non-MVCC persistent variables would probably be prohibitively
expensive to make crash-safe, and seem even more pointless.

Now, I can see shared variables whose state is visible across backends
but is neither MVCC nor persistent being a fun toy, albeit not one I
find likely to be particularly useful personally. But we can probably
already do that in extensions, we've got most if not all of the needed
infrastructure. Because we're a shared-nothing-by-default system, such
variables will probably need shared memory segments that need to be
allocated and, if new vars are added or their values grow too much,
re-allocated. Plus locks to control access. All of which we can
already do. Most of the uses I can think of for such things are met
reasonably well by advisory locking already, and I expect most of the
rest would be met by autonomous commit, so it feels a bit like a
feature looking for a use-case.

So .... lets take a step back or eight and ask "why?"


Pavel:

* Why is it so necessary for plpgsql variables to be implemented as
persistent entities that are in the catalogs in order for you to
achieve the static checking you want to? Is this due to limitations of
your approach in plpgsql_check, or more fundamental issues? Explain.


Fabien:

* What use do you have for persistent-data variables? Please set out
some use cases where they solve problems that are currently hard to to
solve or greatly improve on the existing solutions.

* On what basis do you _oppose_ persistently defining variables in the
catalogs as their own entities? (My own objection is that "temporary
variables" would make our existing catalog bloat issues for temp
objects even worse).

* Do you expect temporary/transient session variables to come into
existence when first set, or to require some form of explicit
definition?


Everyone:

* Does anyone care about or want variables whose value is shared
across sessions? If so, why? Set out use cases.

* Does anyone care about or want variables whose value becomes visible
as soon as set, i.e. non-MVCC? If so, why? Set out use cases.

* Does anyone care about or want variables whose value is persistent
on-disk across restarts and/or crashes, maybe recorded in WAL for
replication, etc? If so, justify how this is better than a relation in
real-world practical terms.



-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services