Update row in trigger without recursion - postgresql

Question

I have two tables:
CREATE TABLE first (
id text primary key,
updated_at timestamp,
data text
);
CREATE TABLE second (
id text REFERENCES first (id),
book_error text,
);
and I need to update updated_at field in first table always, when any of these tables has updated. I wrote this:
CREATE FUNCTION update_timestamp() RETURNS trigger AS $$
BEGIN
UPDATE first
SET updated_at = current_timestamp
WHERE id = NEW.id;
RETURN NEW;
END;
$$ LANGUAGE plpgsql;
DO $$
DECLARE
t text;
BEGIN
FOR t IN
SELECT table_name FROM information_schema.tables
WHERE table_schema = 'public'
LOOP
EXECUTE format('CREATE TRIGGER update_timestamp
BEFORE INSERT OR UPDATE ON %I
FOR EACH ROW EXECUTE PROCEDURE update_timestamp()',
t);
END LOOP;
END;
$$ LANGUAGE plpgsql;
But it's not working because update statement inside my trigger causes call of this trigger again before executing.
How can I do update inside trigger without firing it trigger again?

Answer 1

Per the documentation:
TG_TABLE_NAME
Data type name; the name of the table that caused the trigger invocation.
Use the variable in the trigger function:
CREATE OR REPLACE FUNCTION update_timestamp() RETURNS trigger AS $$
BEGIN
IF TG_TABLE_NAME = 'first' THEN
NEW.updated_at = current_timestamp;
ELSE
UPDATE first
SET updated_at = current_timestamp
WHERE id = NEW.id;
END IF;
RETURN NEW;
END;
$$ LANGUAGE plpgsql;

Answer 2

From Postgresql documentation,
pg_trigger_depth() - current nesting level of PostgreSQL triggers (0
if not called, directly or indirectly, from inside a trigger)
You can use this inside your trigger function to check if it is called from inside trigger
DO $$
DECLARE
t text;
BEGIN
FOR t IN
SELECT table_name FROM information_schema.tables
WHERE table_schema = 'public'
LOOP
EXECUTE format('CREATE TRIGGER update_timestamp
BEFORE INSERT OR UPDATE ON %I
FOR EACH ROW
WHEN (pg_trigger_depth() = 0)
EXECUTE PROCEDURE update_timestamp()',
t);
END LOOP;
END;
$$ LANGUAGE plpgsql;
test=# Insert into first select 1,now(), 'test';
INSERT 0 1
test=# select * from first;
id | updated_at | data
----+----------------------------+------
1 | 2018-10-29 20:18:25.227281 | test
(1 row)
test=# Insert into second select 1, 'test_error';
INSERT 0 1
test=# select * from first;
id | updated_at | data
----+----------------------------+------
1 | 2018-10-29 20:19:07.456737 | test