Re: poc - possibility to write window function in PL languages

Hi

I simplified access to results of winfuncargs functions by proxy type
"typedvalue". This type can hold any Datum value, and allows fast cast to
basic buildin types or it can use (slower) generic cast functions. It is
used in cooperation with a plpgsql assign statement that can choose the
correct cast implicitly. When the winfuncarg function returns a value of
the same type, that is expected by the variable on the left side of the
assign statement, then (for basic types), the value is just copied without
casts. With this proxy type is not necessary to have special statement for
assigning returned value from winfuncargs functions, so source code of
window function in plpgsql looks intuitive to me.

Example - implementation of "lag" function in plpgsql

create or replace function pl_lag(numeric)
returns numeric as $$
declare v numeric;
begin
v := get_input_value_in_partition(windowobject, 1, -1, 'seek_current',
false);
return v;
end;
$$ language plpgsql window;

I think this code is usable, and I assign this patch to commitfest.

Regards

Pavel