Re: Allow any[] as input arguments for sql/plpgsql functions to mimic format()

Hey!

OK - thank you for the update and the explanation.

My reasoning in this case is - if we allow the any[] type to only be
passed to other functions that accept any[], and disallow any kind of
other operations on this array (such as retrieving its elements or
modifying it), I do not yet see any places where it might introduce a
performance regression. These arguments will literally be pass-only, and
since we are unable to interact with them in any other way, there will
be no possibility of type mismatches and therefore for performance
penalties.

This approach puts all the heavy work on the plpgsql compiler - it will
need to ensure that, if there is a any[] or VARIADIC any variable in a
function arglist, it must NOT be accessed in any way, and can only be
passed to other functions which accept any[] or VARIADIC any.

BR
~phoe

On 22.04.2019 12:09, Pavel Stehule wrote:
> Hi
>
> po 22. 4. 2019 v 11:27 odesílatel Michał "phoe" Herda
> <phoe(at)disroot(dot)org <mailto:phoe(at)disroot(dot)org>> napsal:
>
> Hey everyone,
>
> I am writing a plpgsql function that (to greatly simplify) raises an
> exception with a formatted* message. Ideally, I should be able to call
> it with raise_exception('The person %I has only %I bananas.', 'Fred',
> 8), which mimics the format(text, any[]) calling convention.
>
> Here is where I have encountered a limitation of PostgreSQL's design:
> https://www.postgresql.org/docs/11/datatype-pseudo.html mentions
> explicitly that, "At present most procedural languages forbid use of a
> pseudo-type as an argument type".
>
> My reasoning is that I should be able to accept a value of some
> type if
> all I do is passing it to a function that accepts exactly that type,
> such as format(text, any[]). Given the technical reality, I assume
> that
> I wouldn't be able to do anything else with that value, but that is
> fine, since I don't have to do anything with it regardless.
>
> BR
> Michał "phoe" Herda
>
> *I do not want to use the obvious solution of
> raise_exception(format(...)) because the argument to that function is
> the error ID that is then looked up in a table from which the error
> message and sqlstate are retrieved. My full code is in the
> attached SQL
> file. Once it is executed:
>
> SELECT gateway_error('user_does_not_exist', '2'); -- works but is
> unnatural,
> SELECT gateway_error('user_does_not_exist', 2); -- is natural but
> doesn't work.
>
>
> It is known problem, and fix is not easy.
>
> Any expressions inside plpgsql are simple queries like SELECT expr,
> and they are executed same pipeline like queries.
>
> The plans of these queries are stored and reused. Originally these
> plans disallow any changes, now some changes are supported, but
> parameters should be same all time. This is ensured by disallowing
> "any" type.
>
> Other polymorphic types are very static, so there is not described risk.
>
> Probably some enhancement can be in this are. The plan can be
> re-planed after some change - but it can has lot of performance
> impacts. It is long open topic. Some changes in direction to dynamic
> languages can increase cost of  some future optimization to higher
> performance :-(.
>
> Regards
>
> Pavel
>
>
>
>