Constraints involving a system table

Hello,

I would like to create a constraint that involves the system table.

CREATE TABLE metadata_tables
(
   schemaname text REFERENCES pg_namespace(nspname) ON DELETE CASCADE
CONSTRAINT valid_schema CHECK (schemaname != 'public' AND schemaname !=
'information_schema' AND schemaname NOT LIKE 'pg_%'),
   tablename text,
   source text
) WITH (OIDS=FALSE)
;

Basically, I only want to allow that the user can use schemanames present
in the current database as input in the column schemaname.

When I try to create this table, I get an error that it is a system table.

ERROR:  permission denied: "pg_namespace" is a system catalog

However, when looking at the system table definition, it has "GRANT SELECT
ON TABLE pg_namespace TO public;" - so everybody should be able to query a
system table.

Why is this not working? What other privileges besides "SELECT" would one
need to define a constraint?

Thanks for any hint,
Andreas

--
Andreas Neumann
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