Custom function ROWS hint ignored due to inlining?

[Philip Semanchuk <philip@xxxxxxxxxxxxxxxxxxxxx>]

Hi,
I have a custom function where the ROWS hint is getting ignored. I think it’s because the function is getting inlined, but I’d like a second opinion.

Here’s my working (contrived) example.

CREATE TABLE my_table (
    id int primary key GENERATED ALWAYS AS IDENTITY,
    base_value int NOT NULL
);

INSERT INTO my_table (base_value) VALUES (42);

CREATE OR REPLACE FUNCTION fn_get_deltas(base_value int)
RETURNS TABLE (delta int, total int) AS $$
    SELECT 
        generate_series(1, CASE base_value WHEN 42 THEN 4 ELSE 20 END), 
        base_value + generate_series(1, CASE base_value WHEN 42 THEN 4 ELSE 20 END)
$$ LANGUAGE sql IMMUTABLE PARALLEL SAFE ROWS 10;

EXPLAIN
SELECT base_value, delta, total
FROM my_table
CROSS JOIN LATERAL (SELECT delta, total FROM fn_get_deltas(base_value)) AS foo
+------------------------------------------------------------------+
| QUERY PLAN                                                       |
|------------------------------------------------------------------|
| Nested Loop  (cost=0.00..107427.80 rows=2260000 width=12)        |
|   ->  Seq Scan on my_table  (cost=0.00..32.60 rows=2260 width=4) |
|   ->  Result  (cost=0.00..27.52 rows=1000 width=8)               |
|         ->  ProjectSet  (cost=0.00..5.02 rows=1000 width=4)      |
|               ->  Result  (cost=0.00..0.01 rows=1 width=0)       |
+------------------------------------------------------------------+

The plan estimates 1000 rows from the CROSS JOIN despite the “ROWS 10” hint on my function. I think this is because the planner never sees fn_get_deltas() — it has been inlined by the query preprocessor because fn_get_deltas() meets the criteria for inlining (https://wiki.postgresql.org/wiki/Inlining_of_SQL_functions). Instead of 10 rows, the planner uses its default assumption of 1000 rows.

If I change the function to VOLATILE to prevent inlining, I get this plan.
+-------------------------------------------------------------------------+
| QUERY PLAN                                                              |
|-------------------------------------------------------------------------|
| Nested Loop  (cost=0.25..484.85 rows=22600 width=12)                    |
|   ->  Seq Scan on my_table  (cost=0.00..32.60 rows=2260 width=4)        |
|   ->  Function Scan on fn_get_deltas  (cost=0.25..0.35 rows=10 width=8) |
+-------------------------------------------------------------------------+

I would prefer to have the function inlined for better performance, but I can declare it VOLATILE if that’s necessary to give decent estimates to the planner. Am I correctly reading the situation? If so, is there another solution that allows inlining *and* making the ROWS hint visible to the planner?

Thanks a bunch
Philip