Re: extensions are hitting the ceiling

On Tue, Apr 16, 2019 at 4:24 AM Eric Hanson <eric(at)aquameta(dot)com> wrote:

>
>
> On Tue, Apr 16, 2019 at 12:47 AM Noah Misch <noah(at)leadboat(dot)com> wrote:
>
>> On Mon, Mar 18, 2019 at 09:38:19PM -0500, Eric Hanson wrote:
>> > I have heard talk of a way to write extensions so that they dynamically
>> > reference the schema of their dependencies, but sure don't know how that
>> > would work if it's possible. The @extschema@ variable references the
>> > *current* extension's schema, but not there is no dynamic variable to
>> > reference the schema of a dependency.
>>
>> If desperate, you can do it like this:
>>
>> DO $$ BEGIN EXECUTE format('SELECT %I.earth()',
>> (SELECT nspname FROM pg_namespace n
>> JOIN pg_extension ON n.oid = extnamespace
>> WHERE extname = 'earthdistance' )); END $$;
>>
>> Needless to say, that's too ugly. Though probably unimportant in
>> practice, it
>> also has a race condition vs. ALTER EXTENSION SET SCHEMA.
>>
>> > Also it is possible in theory to dynamically set search_path to contain
>> > every schema of every dependency in play and then just not specify a
>> schema
>> > when you use something in a dependency. But this ANDs together all the
>> > scopes of all the dependencies of an extension, introducing potential
>> for
>> > collisions, and is generally kind of clunky.
>>
>> That's how it works today, and it has the problems you describe. I
>> discussed
>> some solution candidates here:
>>
>> https://www.postgresql.org/message-id/20180710014308.GA805781@rfd.leadboat.com
>>
>> The @DEPNAME_schema@ thing was trivial to implement, but I shelved it.
>> I'm
>> attaching the proof of concept, for your information.
>>
>
> Interesting.
>
> Why shelved? I like it. You said you lean toward 2b in the link above,
> but there is no 2b :-) but 1b was this option, which maybe you meant?
>
> The other approach would be to have each extension be in it's own schema,
> whose name is fixed for life. Then there are no collisions and no
> ambiguity about their location. I don't use NPM but was just reading
> about how they converted their package namespace from a single global
> namespace with I think it was 30k packages in it,
> to @organization/packagename. I don't know how folks would feel about a
> central namespace registry, I don't love the idea if we can find a way
> around it, but would settle for it if there's no better solution. Either
> that or use a UUID as the schema name. Truly hideous. But it seems like
> your approach above with just dynamically looking up the extension's schema
> as a variable would solve everything.
>
> There is the problem of sequencing, where extension A installs dependency
> extension B in it's own schema. Then extension C also wants to use
> dependency B, but extension A is uninstalled and extension B is now still
> hanging around in A's old schema. Not ideal but at least everything would
> still function.
>
> I'll keep thinking about it...
>

We would probably be wise to learn from what has gone (so I hear) terribly
wrong with the Node / NPM packaging system (and I'm sure many before it),
namely versioning. What happens when two extensions require different
versions of the same extension? At a glance it almost seems unsolvable,
given the constraint that an extension can only be installed once, and only
at a single version. I don't understand why that constraint exists though.

Eric