Re: Allow any[] as input arguments for sql/plpgsql functions to mimic format() - Mailing list pgsql-hackers
| From | Pavel Stehule |
|---|---|
| Subject | Re: Allow any[] as input arguments for sql/plpgsql functions to mimic format() |
| Date | |
| Msg-id | CAFj8pRBAY3YL90PM+bwAQ0+shhQF-tL76ZBDRQF_FsznJ95nPw@mail.gmail.com Whole thread Raw |
| In response to | Re: Allow any[] as input arguments for sql/plpgsql functions to mimicformat() (Michał "phoe" Herda <phoe@disroot.org>) |
| List | pgsql-hackers |
Hi
po 22. 4. 2019 v 19:20 odesílatel Michał "phoe" Herda <phoe@disroot.org> napsal:
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.
PLpgSQL compiler knows nothing about a expressions - the compiler process only plpgsql statements. Expressions are processed at runtime only by SQL parser and executor.
It is good to start with plpgsql codes - https://github.com/postgres/postgres/tree/master/src/pl/plpgsql/src
you can see there, so plpgsql is very different from other compilers. It just glue of SQL expressions or queries, that are black box for PLpgSQL compiler and executor.
Just any[] is not plpgsql way. For your case you should to use a overloading
create or replace function fx(fmt text, par text)
returns void as $$
begin
raise notice '%', format(fmt, par);
end;
$$ language plpgsql;
create or replace function fx(fmt text, par numeric)
returns void as $$
begin
raise notice '%', format(fmt, par);
end;
$$ language plpgsql;
There is another limit, you cannot to declare function parameter type that enforce explicit casting
can be nice (but it is strange idea) to have some other flags for arguments
CREATE OR REPLACE FUNCTION gateway_error(fmt text, par text FORCE EXPLICIT CAST)
...
Regards
Pavel
BR
~phoeOn 22.04.2019 12:09, Pavel Stehule wrote:Hipo 22. 4. 2019 v 11:27 odesílatel Michał "phoe" Herda <phoe@disroot.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 :-(.RegardsPavel
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