In postgres 15, how can we define constants that pertain to business logic rather than db configuration settings? I'd like to define them within sql scripts rather than passed through the `psql -v MY_CONSTANT=123` command & I don't care if they persist only for the session.

In prior versions I quietly got away with the following syntax:
```sql
SET MY_CONSTANT TO 123;     -- declared
CALL my_func(:MY_CONSTANT); -- used
```
Which from the [documentation](https://www.postgresql.org/docs/15/sql-set.html) seems to be intended for modifying known db config settings & explains why I am encountering issues like this:
```
my_script.sql:2: ERROR:  unrecognized configuration parameter "my_constant"
```