Snowflake: Decimal or null input to function results in "Unsupported subquery type" - sql

Question

Given the following function:
CREATE
OR REPLACE FUNCTION myfunction(a float, b float, c float)
RETURNS float AS
$$
select sum(1/(1+exp(-(series - c)/4)))
from (
select (a + ((row_number()) over(order by 0))*1) series
from table(generator(rowcount => 10000)) x
qualify series <= b
)
$$;
I get all the expected results when executing the following queries:
select
myfunction(1, 10, 1);
select
myfunction(1, 100, 1);
select
myfunction(1, 10, 1.1);
select
myfunction(0, 1, 89.87);
select
myfunction(0, 1, null);
However when I run the following query:
select
myfunction(a, b, c)
from
(
select
1 as a,
10 as b,
1.1 as c
union
select
0 as a,
1 as b,
null as c
);
I get an error:
"Unsupported subquery type cannot be evaluated".
While this query does work:
select
a, b, myfunction(a, b, c)
from
(
select
1 as a,
10 as b,
1 as c
union
select
1 as a,
100 as b,
1 as c
);
Why can't Snowflake handle null or decimal numbers in the 'c' column when I input multiple rows while individual rows weren't a problem?
And how can this function be rewritten to be able to handle these cases?

Answer 1

SQL UDFs are converted to subqueries (for now), and if Snowflake can not determine the data type returned from these subqueries, you get the "Unsupported subquery" error. The issue is not about decimals or null. The issue is A and C variables (which are used in SUM()) contain different values. For example, the following ones work:
select
myfunction(a, b, c )
from
(
select
1 as a,
1 as b,
1.1 as c
union
select
1 as a,
100 as b,
1.1 as c
);
select
myfunction(a, b, c )
from
(
select
1 as a,
1 as b,
null as c
union
select
1 as a,
100 as b,
null as c
);
You may hit these kinds of errors when you try to write complex functions with SQL UDFs. Sometimes rewriting them can help, but I don't see a way for this one. As a workaround, you may re-write it in JavaScript because JS UDFs are not converted to subqueries:
CREATE
OR REPLACE FUNCTION myfunction(a float, b float, c float)
RETURNS float
language javascript AS
$$
var res = 0.0;
for (let series = A + 1; series <= B; series++) {
res += (1/(1+Math.exp(-(series - C)/4)));
}
return res;
$$;
According to my tests, the above UDF returns the same result as the SQL version, and it doesn't hit "Unsupported subquery" error.

Answer 2

Weird one. Can you try selecting from the subquery and running it through a cast?
Like this:
select a, b, c
from
(select cast(a as float) as a, cast(b as float) as b, cast(c as float) as c from
(
select
1 as a,
10 as b,
1 as c
union
select
1 as a,
100 as b,
null as c
) as t) as x

Answer 3

In the end implementing it as a python function allowed for also handling all the edge cases:
CREATE
OR REPLACE FUNCTION myfunction(a float, b float, c float)
returns float
language python
runtime_version=3.8
handler='compute'
as
$$
def compute(a, b, c):
import math
if b < a:
return None
if c is None:
return None
res = []
step_size = 1
it = a
while it < b:
res.append(it)
it += step_size
res = sum([1/(1+math.exp(-1*(i-c)/4)) for i in res])
return res
$$;