Postgresql – Any ideas what is wrong with this code, Error at or near Insert on a new function

There will be no difference in performance between a view and a straight-up query. View is basically a saved query text. If you expect to call this exact query in multiple places in your code, it may make sense to create a view. Otherwise, put a query in your pgplsql code, it will improve readability of it.

As I know there is no possibility to restrict access to the DB metadata at least for reading in PostgreSQL.

As an alternative you can to create another DB for login(s) and use foreign-data wrapper to access to the desired object(s) in the your main DB.

Here is a simple scenario (not fully tested):

Login to the main DB and create test table:

create table t (x int);

Create another DB for login(s):

create database logindb with owner = postgres;
grant all on database logindb to postgres;

Create user:

create role testrole password '111' login nosuperuser noinherit;
revoke connect on database postgres from testrole;

Now login to the newely created DB with user postgres and create FDW:

create extension postgres_fdw;
create server myserver foreign data wrapper postgres_fdw options (host 'localhost', dbname 'postgres', port '5432');
create user mapping for postgres server myserver options(user 'postgres', password '111');
create user mapping for testrole server myserver options(user 'postgres', password '111');
create foreign table t(x int) server myserver options(schema_name 'public', table_name 't');
grant insert on t to testrole;

Theoretically thats all. You can use testrole to connect to the logindb and to insert data into the table t in the main DB postgres. And 'testrole' can not to connect to the main DB and see anything in it.