passing variable while do insert operation

passing_var_while_insert.sql

/*
https://dba.stackexchange.com/questions/303502/pass-a-variable-with-insert-update-delete-statements/303546#303546
https://dbfiddle.uk/ZsFssnOW 
 */

CREATE TABLE ins_tbl (
    id int GENERATED ALWAYS AS IDENTITY,
    name text
);

CREATE VIEW v_ins_tbl AS
SELECT
    *,
    NULL::int AS site_id
FROM
    ins_tbl;

CREATE OR REPLACE FUNCTION trg_ins_tbl ()
    RETURNS TRIGGER
    LANGUAGE plpgsql
    AS $func$
BEGIN
    CASE NEW.site_id
    WHEN 1 THEN
        INSERT INTO ins_tbl (name)
            VALUES (NEW.name); RETURN new;
    WHEN 2 THEN
        INSERT INTO ins_tbl (name)
            VALUES (NEW.name); RETURN NULL;
    WHEN 0 THEN
            RETURN NULL;
    ELSE
        RAISE EXCEPTION 'unexpected site_id: %', NEW.site_id;
        END CASE;
END
$func$;

CREATE TRIGGER trg_insbef_ins_tbl
    INSTEAD OF insert ON v_ins_tbl FOR EACH ROW
    EXECUTE FUNCTION trg_ins_tbl ();

-- missing or unexpected site_id raises custom exception
INSERT INTO v_ins_tbl (name)
    VALUES ('xxx');

-- missing or unexpected site_id raises custom exception
INSERT INTO v_ins_tbl (name, site_id)
    VALUES ('xxx', 7);
/*
ERROR:  unexpected site_id: 7
CONTEXT:  PL/pgSQL function trg_ins_tbl() line 13 at RAISE
 */

-- propagated normally
INSERT INTO v_ins_tbl (name, site_id)
    VALUES ('bar___1', 1)
RETURNING
    *;

-- INSERT is propagated, but not reported and Postgres stops there
INSERT INTO v_ins_tbl (name, site_id)
    VALUES ('foo', 2)
RETURNING
    *;

TABLE ins_tbl;

-- INSERT cancelled silently
INSERT INTO v_ins_tbl (name, site_id)
    VALUES ('xxx', 0)
RETURNING
    *;

--normal way.
INSERT INTO v_ins_tbl (id, name, site_id)
    VALUES (DEFAULT, 'test', 1)
RETURNING
    *;

TABLE ins_tbl;