PostgreSQL C function to get values - c

Question

I am trying to write a PostgreSQL function in C.
My goal is finding minimum value of a list. So, my function will be executed like these:
SELECT min_to_max(val) FROM (VALUES(1),(2),(3)) x(val);
SELECT min_to_max(val) FROM my_table;
Here is my C code and I lost here. For example, there is a function called "PG_GETARG_INT32" to get integer values, but I don' t know how to get values from a table in order to process. Any idea?
#include "postgres.h"
#include "fmgr.h"
PG_MODULE_MAGIC;
PG_FUNCTION_INFO_V1(get_sum);
Datum
get_sum(PG_FUNCTION_ARGS)
{
ArrayType *v1,
bool isnull;
isnull = PG_ARGISNULL(0);
if (isnull)
ereport( ERROR,
( errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("The input cannot be empty")));
List a = PG_GETARR_SOMEFUNCTION_2_GET_LIST(0);
# for loop iteration to find min_val
# return min_val
}
Edited(2022.05.13 - below is edited part):
Thanks to #Laurenz Albe. I made some progress.
Yet, now I want to go further(No needs to be in C language. As Laurenz Albe stated, I am just taking small steps).
My functions and aggregates like below to find min and max:
CREATE or replace FUNCTION find_min_func(
state integer,
next integer
) RETURNS integer
LANGUAGE plpgsql
STRICT
AS $$
declare
min_val integer;
begin
if $1 <= $2 then min_val := $1;
elsif $2 <$1 then min_val := $2;
end if;
return min_val;
END;
$$;
CREATE or replace AGGREGATE find_min(integer)
(
SFUNC = find_min_func,
STYPE = integer
);
CREATE or replace FUNCTION find_max_func(
state integer,
next integer
) RETURNS integer
LANGUAGE plpgsql
STRICT
AS $$
declare
max_val integer;
begin
if $1 >= $2 then max_val := $1;
elsif $2 > $1 then max_val := $2;
end if;
return max_val;
END;
$$;
CREATE or replace AGGREGATE find_max(integer)
(
SFUNC = find_max_func, -- State function
STYPE = integer -- State type
);
They are working great but now I want to do something like
SELECT min_to_max(val) FROM (VALUES(1),(2),(3)) x(val);
Expected output:
1 -> 3
So, I just wrote a state function and aggregate pair like below(I know it is wrong):
CREATE or replace FUNCTION find_min_and_max_func(
state integer,
next integer
) RETURNS varchar
LANGUAGE plpgsql
STRICT
AS $$
declare
min_val integer;
max_val integer;
output varchar;
begin
if $1 <= $2 then min_val := $1; max_val := $2;
elsif $2 <$1 then min_val := $2; max_val := $1;
end if;
output = cast(min_val as varchar) || '->' || cast(max_val as varchar) ;
return output;
END;
$$;
CREATE or replace AGGREGATE find_min_and_max(integer)
(
SFUNC = find_min_and_max_func, -- State function
STYPE = varchar -- State type
);
It is wrong because state function is taking arguments as integer but returns(?) varchar, so it varying.
How can I arrange my state function here?
Thanks!

Answer 1

To create an aggregate function, you have to use CREATE AGGREGATE. You create that in SQL. What you may need to implement in C is the state transition function (SFUNC), perhaps also others, depending on the kind of aggregate you want to create.
The aggregate function does not have to read from the table; the PostgreSQL executor will feed it the data it needs.
If you start writing C functions, you should perhaps start with something simpler than aggregate functions.

Answer 2

With the help of #Laurenz Albe(thanks to him again), I found the solution. Also, I checked out:
https://hoverbear.org/blog/postgresql-aggregates-with-rust/
https://hashrocket.com/blog/posts/custom-aggregates-in-postgresql
Here is my solution:
CREATE or replace FUNCTION find_min_and_max_func(
state point,
next integer
) RETURNS point
LANGUAGE plpgsql
STRICT
AS $$
declare
min_val integer;
max_val integer;
begin
if state[0] <= next then min_val := state[0];
elsif next < state[0] then min_val := next;
end if;
if state[1] >= next then max_val := state[1];
elsif next > state[1] then max_val := next;
end if;
return point(min_val, max_val) ;
END;
$$;
CREATE or replace FUNCTION find_min_and_max_final_func(
state point
) RETURNS varchar
LANGUAGE plpgsql
STRICT
AS $$
begin
return cast(state[0] as varchar) || '->' || cast(state[1] as varchar) ;
END;
$$;
CREATE or replace AGGREGATE find_min_and_max(integer)
(
SFUNC = find_min_and_max_func, -- State function
STYPE = point, -- State type
FINALFUNC = find_min_and_max_final_func,
initcond = '(1231231232131,0)'
);
SELECT find_min_and_max(value) FROM UNNEST(ARRAY [1, 2, 3]) as value;
find_min_and_max
------------------
1->6
(1 row)
Thanks!