Passing a procedure arguments to a trigger - postgresql

Question

I have a table say:
CREATE TABLE comment (
comment_id SERIAL PRIMARY KEY,
content varchar
);
and another:
CREATE TABLE comment_tree (
ancestor integer NOT NULL,
descendant integer NOT NULL,
PRIMARY KEY (ancestor, descendant),
FOREIGN KEY (ancestor) REFERENCES comment (comment_id),
FOREIGN KEY (descendant) REFERENCES comment (comment_id)
);
Now I have a procedure which insert a new comment as a child of another comment:
CREATE OR REPLACE PROCEDURE add_comment(content varchar, parent_id integer) AS $$
DECLARE
cid integer;
BEGIN
INSERT INTO comment (content) VALUES (content)
RETURNING comment_id INTO cid;
INSERT INTO comment_tree (ancestor, descendant)
VALUES (parent_id, cid);
END;
$$ LANGUAGE plpgsql;
But I want to know if it is possible to pass the parent_id to a trigger when I call the procedure instead of inserting into comment_tree inside the procedure?

Answer 1

You cannot pass an arbitrary variable directly to a trigger. You can use a temporary table that simulate a kind of pipe. Create a temporary table with the value you want to pass in the procedure:
CREATE OR REPLACE PROCEDURE add_comment(content varchar, parent_id integer) AS $$
DECLARE
pid integer;
BEGIN
CREATE TEMP TABLE my_temp_table AS SELECT parent_id;
INSERT INTO comment (content) VALUES (content)
RETURNING comment_id INTO pid;
DROP TABLE my_temp_table;
END;
$$ LANGUAGE plpgsql;
Then you can use it in the trigger fired after insertion:
CREATE OR REPLACE FUNCTION after_insert_on_comment()
RETURNS TRIGGER LANGUAGE plpgsql AS $$
BEGIN
INSERT INTO comment_tree (ancestor, descendant)
SELECT parent_id, new.comment_id
FROM my_temp_table;
RETURN null;
END $$;
Test it in db<>fiddle.