Re: Handling changes to default type transformations in PLs

Jim Nasby <Jim(dot)Nasby(at)BlueTreble(dot)com> writes:
> Some of our PLs have the unfortunate problem of making a weak effort
> with converting types to and from the PL and Postgres. For example,
> plpythonu will correctly transform a complex type to a dict and an array
> to a list, but it punts back to text for an array contained inside a
> complex type. I know plTCL has similar issues; presumably other PLs are
> affected as well.

> While it's a SMOC to fix this, what's not simple is the backwards
> compatability: users that are currently using these types are expecting
> to be handed strings created by the type's output function, so we can't
> just drop these changes in without breaking user code.

> It might be possible to work around this with TRANSFORMs, but that's
> just ugly: first you'd have to write a bunch of non-trivial C code, then
> you'd need to forever remember to specify TRANSFORM FOR TYPE blah.

> Some ways to handle this:

> 1) Use a PL-specific GUC for each case where we need backwards
> compatibility. For plpython we'd need 2. plTCL would need 1 or 2.

> 2) Use a single all-or-nothing GUC. Downside is that if we later decide
> to expand automatic conversion again we'd need yet another GUC.

> 3) Add the concept of PL API versions. This would allow users to specify
> what range of API versions they support. I think this would have been
> helpful with the plpython elog() patch.

> 4) Create a mechanism for specifying default TRANSFORMs for a PL, and
> essentially "solve" these issues by supplying a built-in transform.

> I think default transforms (4) are worth doing no matter what. Having to
> manually remember to add potentially multiple TRANSFORMs is a PITA. But
> I'm not sure TRANSFORMS would actually fix all issues. For example, you
> can't specify a transform for an array type, so this probably wouldn't
> work for one of the plpython problems.

> 3 is interesting, but maybe it would be bad to tie multiple unrelated
> API changes together.

> So I'm leaning towards 1. It means potentially adding a fair number of
> new GUCs, but these would all be custom GUCs, so maybe it's not that
> bad. The other downside to GUCs is I think it'd be nice to be able to
> set this at a schema level, which you can't currently do with GUCs.

> Thoughts?

I think harsh experience has taught us to distrust GUCs that change
code semantics. So I'm not very attracted by option #1, much less
option #2. I'm not sure about option #4 --- it smells like it would
have the same problems as a GUC, namely that it would be
action-at-a-distance on the semantics of a PL function's arguments,
with insufficient ability to control the scope of the effects.

So that leaves #3, which doesn't seem all that unreasonable from here.
We don't have a problem with bundling a bunch of unrelated changes
into any one major PG revision. The scripting languages we're talking
about calling do similar things. So why not for the semantics of the
glue layer?

It seems like you really need to be able to specify this at the
per-function level, which makes me think that specifying
"LANGUAGE plpython_2" or "LANGUAGE plperl_3" might be the right
kind of API.

regards, tom lane