Why is my function inlined only when STABLE?

Hi all,
I have a function that isn't being inlined, and I would appreciate help to understand why that's the case. 

I'm using PG 11.15. I know that if I declare my function IMMUTABLE and it calls a non-IMMUTABLE function, Postgres won't inline my function. But even when my function calls only substring() (which I understand to be IMMUTABLE based on '\df+ substring'), I still can't get Postgres to inline it. If I re-declare my function as STABLE, then Postgres inlines it. According to the rules I understand (https://wiki.postgresql.org/wiki/Inlining_of_SQL_functions#Inlining_conditions_for_scalar_functions), the IMMUTABLE version of my function should be inlined too. What am I missing?

Here's a log of a CLI session showing that the IMMUTABLE version is not inlined, but the STABLE one is.


show track_functions
+-------------------+
| track_functions   |
|-------------------|
| all               |
+-------------------+
SHOW
me@/tmp:wylan#
SELECT * FROM pg_stat_user_functions
+----------+--------------+------------+---------+--------------+-------------+
| funcid   | schemaname   | funcname   | calls   | total_time   | self_time   |
|----------+--------------+------------+---------+--------------+-------------|
+----------+--------------+------------+---------+--------------+-------------+
SELECT 0
Time: 0.021s
me@/tmp:wylan#
CREATE OR REPLACE FUNCTION f(foo text)
 RETURNS text
 AS $$
     SELECT substring(foo FROM 1 FOR 2)
 $$ LANGUAGE sql IMMUTABLE PARALLEL SAFE;
CREATE FUNCTION
Time: 0.003s
me@/tmp:wylan#
select f('4242')
+-----+
| f   |
|-----|
| 42  |
+-----+
SELECT 1
Time: 0.008s
me@/tmp:wylan#
SELECT * FROM pg_stat_user_functions
+----------+--------------+------------+---------+--------------+-------------+
| funcid   | schemaname   | funcname   | calls   | total_time   | self_time   |
|----------+--------------+------------+---------+--------------+-------------|
| 14472085 | public       | f          | 1       | 0.05         | 0.05        |
+----------+--------------+------------+---------+--------------+-------------+
SELECT 1
Time: 0.022s
me@/tmp:wylan#
DROP FUNCTION f(text)
DROP FUNCTION
Time: 0.001s
me@/tmp:wylan#
CREATE OR REPLACE FUNCTION f(foo text)
 RETURNS text
 AS $$
     SELECT substring(foo FROM 1 FOR 2)
 $$ LANGUAGE sql STABLE PARALLEL SAFE;
CREATE FUNCTION
Time: 0.003s
me@/tmp:wylan#
select pg_stat_reset()
+-----------------+
| pg_stat_reset   |
|-----------------|
|                 |
+-----------------+
SELECT 1
Time: 0.008s
me@/tmp:wylan#
SELECT * FROM pg_stat_user_functions
+----------+--------------+------------+---------+--------------+-------------+
| funcid   | schemaname   | funcname   | calls   | total_time   | self_time   |
|----------+--------------+------------+---------+--------------+-------------|
+----------+--------------+------------+---------+--------------+-------------+
SELECT 0
Time: 0.022s
me@/tmp:wylan#
select f('4242')
+-----+
| f   |
|-----|
| 42  |
+-----+
SELECT 1
Time: 0.008s
me@/tmp:wylan#
SELECT * FROM pg_stat_user_functions
+----------+--------------+------------+---------+--------------+-------------+
| funcid   | schemaname   | funcname   | calls   | total_time   | self_time   |
|----------+--------------+------------+---------+--------------+-------------|
+----------+--------------+------------+---------+--------------+-------------+
SELECT 0
Time: 0.019s
me@/tmp:wylan#




Thanks
Philip