Hey,
I've noticed that the current documentation doesn't mention IF EXISTS clause for ALTER STATISTICS in the synopsis section, where PG supports it.
(Only for the last item, that is ALTER STATISTICS .. SET STATISTICS; for the others, PG just throws a syntax error.)
I'm from the Citus team and noticed this while bug fixing, and I wonder if it is intentional or not. If it's intentionally supported while the other ALTER STATISTICS statement types are not supported, it would be good to mention that in the documentation.