test=# \d nsi.nsi_dictionary;
                                           Table "nsi.nsi_dictionary"
   Column    |           Type           | Collation | Nullable |                    Default
-------------+--------------------------+-----------+----------+------------------------------------------------
 id          | integer                  |           | not null | nextval('nsi.nsi_dictionary_id_seq'::regclass)
 change_date | timestamp with time zone |           | not null |
 nsi_value   | text                     |           |          |
Indexes:
    "nsi_dictionary_pkey" PRIMARY KEY, btree (id)

CREATE OR REPLACE FUNCTION nsi.add_data_nsi (text) RETURNS timestamp with time zone AS '
DECLARE
nsi_value ALIAS FOR $1;
right_now timestamp;
BEGIN
right_now := ''now'';
INSERT INTO nsi.nsi_dictionary VALUES (default, right_now, nsi_value);
RETURN right_now;
END;
'
LANGUAGE 'plpgsql'
SECURITY DEFINER
COST 10;

test=# create role client;
CREATE ROLE
test=# alter role client with login;
ALTER ROLE
postgres=# GRANT CONNECT ON DATABASE test TO client;
GRANT
test=# grant execute on function nsi.add_data_nsi to client;
GRANT

 Result  (cost=0.00..0.04 rows=1 width=8) (actual time=0.161..0.162 rows=1 loops=1)
   Buffers: shared hit=3
 Planning Time: 0.031 ms
 Execution Time: 0.182 ms
(4 rows)