Postgres trigger to update records of several parent tables based on values of a many-to-many join table referencing those parent table records

I have a PostgreSQL 15 join table schema.m2mjointable to link records of several parent tables (schema.table_a, schema.table_b, ...) with other records of those same parent tables: | id | ref_table | ref_id | rep_table | rep_id | |---------:|-------------:|-------:|-------------:|-------:| | IDENTITY | TEXT | INT | TEXT | INT | |----------|--------------|--------|--------------|--------| | 1 | table_a | 1 | table_a | 2 | | 2 | table_a | 1 | table_c | 3 | | 3 | table_a | 4 | table_b | 7 | | 4 | table_b | 5 | table_a | 8 | | 5 | table_b | 9 | table_b | 11 | | 6 | table_c | 7 | table_b | 11 | In the parent tables (schema.table_a, schema.table_b, ...), there is always a boolean field named referenced which should tell if the record i of table n is at least referenced *once* by the pair of columns ref_table and ref_id in this join table. If that's the case it should be set to TRUE. It should be set to FALSE if it's never present in this pair of columns. How could I build a trigger function to update the referenced field of the parent tables with such trigger:

CREATE TRIGGER schema.set_referenced_state_biudt
  BEFORE INSERT OR UPDATE OR DELETE
  ON schema.m2mjointable
  FOR EACH ROW EXECUTE PROCEDURE schema.set_referenced_state();

BEGIN;
CREATE OR REPLACE FUNCTION schema.set_referenced_state()
  RETURNS TRIGGER 
  LANGUAGE PLPGSQL
  AS
$$
BEGIN
   -- set the BOOL value of the "referenced" field to TRUE or FALSE
   -- in the record id=value_of(ref_id) of the parent table=value_of(ref_table) 
   -- corresponding to those values of the record being currently inserted/updated/deleted
END;
$$;