[Firebird-devel] paramdsc

[Dmitry K. <di...@de...>]

Hello, All!

I'm trying to test ParamDSC mechanism with Delphi, and got strange things:
(Delphi code is not a question, I'm trying to understand paramdsc itself).

I have function declared as

declare external function invl
int by descriptor, int by descriptor
returns int by descriptor
entry_point 'iNvl' module_name 'dscexam'

and
function iNvl(var p1, p2: paramdsc): pparamdsc; cdecl; export;
begin
  if p1.dsc_flags = 1 then
// if p1 <> nil then
   Result:=@p2
  else
   Result:=@p1;
end;

exactly the same as
FBUDF_API paramdsc* iNvl(paramdsc* v, paramdsc* v2)
by Claudio Valderrama.

it works fine except one case - when I pass expression as a value.

Example

select iNvl(rating, -1) from customers

works OK. If RATING is not null, then RATING is returned. If
RATING is null, then -1 returned.

But, If I call

select iNvl(rating, rating +1) from customers
then I got p2 as nil (null) passed into function when RATING is null!
I think it is not correct because paramdsc must be passed anyway, but
with NULL "value". Am I wrong?

If I'm wrong, we have 2 cases:
1. when null value is passed inside variable or parameter  paramdsc is filled OK
2. when null value is passed itself - paramdsc address is 0 (nil, null or whatever).

Also I've found if this udf returns pointer to nil (null, 0), select will
return 0, not null value. I.e. if we want to return null value we must
fill correctly paramdsc? I'm confused...

Also I've found that if I call

select iNvl(rating + 1, rating) from customers
that rating + 1 will be INT64 type. But if rating+1 will be null,
rating value will be returned, and it will be INTEGER.
Isn't it dangerous to mix different data types in one column returned to client?
What will happen if first returned UDF value will be integer and all another int64?

p.s. excuse me for a lot of questions but now BY DESCRIPTOR seems very misty not
only for me...

-- 
Dmitry Kuzmenko, Epsylon Technologies.