poc - possibility to write window function in PL languages

Hi

I wrote a proof concept for the support window function from plpgsql.

Window function API - functions named WinFuncArg* are polymorphic and it is
not easy to wrap these functions for usage from SQL level. I wrote an
enhancement of the GET statement - for this case GET WINDOW_CONTEXT, that
allows safe and fast access to the result of these functions.

Custom variant of row_number can look like:

create or replace function pl_row_number()
returns bigint as $$
declare pos int8;
begin
pos := get_current_position(windowobject);
pos := pos + 1;
perform set_mark_position(windowobject, pos);
return pos;
end
$$
language plpgsql window;

Custom variant of lag function can look like:

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

Custom window functions can be used for generating missing data in time
series

create table test_missing_values(id int, v integer);
insert into test_missing_values
values(1,10),(2,11),(3,12),(4,null),(5,null),(6,15),(7,16);

create or replace function pl_pcontext_test(numeric)
returns numeric as $$
declare
n numeric;
v numeric;
begin
perform get_input_value_for_row(windowobject, 1);
get pg_window_context v = PG_INPUT_VALUE;
if v is null then
v := get_partition_context_value(windowobject, null::numeric);
else
perform set_partition_context_value(windowobject, v);
end if;
return v;
end
$$
language plpgsql window;

select id, v, pl_pcontext_test(v) over (order by id) from
test_missing_values;
id | v | pl_pcontext_test.
----+----+------------------
1 | 10 | 10
2 | 11 | 11
3 | 12 | 12
4 | | 12
5 | | 12
6 | 15 | 15
7 | 16 | 16
(7 rows)

I think about another variant for WinFuncArg functions where polymorphic
argument is used similarly like in get_partition_context_value - this patch
is prototype, but it works and I think so support of custom window
functions in PL languages is possible and probably useful.

Comments, notes, ideas, objections?

Regards

Pavel