From the docs: https://www.postgresql.org/docs/9.6/static/sql-createtable.html "Currently, CHECK expressions cannot contain subqueries nor refer to variables other than columns of the current row. The system column tableoid may be referenced, but not any other system column.
I wonder if that should say "should not," or be followed by something like this:
Make it say "must not" and I'd agree to change the word "cannot" and leave the rest. Adding a note regarding functions seems appropriate.
Aside from being a bit more verbose there is nothing useful that writing this as "CHECK function()" provides that you don't also get by writing "CREATE TRIGGER". In a green field we'd probably lock down CHECK a bit more but there is too much code that is technically wrong but correctly functioning that we don't want to break. IOW, we cannot mandate that the supplied function be immutable even though we should. And we don't even enforce immutable execution if a function is defined that way.
