Prevent and/or detect cycles in postgres

Question

Assuming a schema like the following:

CREATE TABLE node (
  id       SERIAL PRIMARY KEY,
  name     VARCHAR,
  parentid INT REFERENCES node(id)
);
         


        

Answer 1

CREATE OR REPLACE FUNCTION detect_cycle()
  RETURNS TRIGGER AS
$func$
DECLARE
  cycle int[];
BEGIN
EXECUTE format('WITH RECURSIVE search_graph(%4$I, path, cycle) AS (
  SELECT g.%4$I, ARRAY[g.%3$I, g.%4$I], (g.%3$I = g.%4$I)
    FROM %1$I.%2$I g
   WHERE g.%3$I = $1.%3$I
  UNION ALL
  SELECT g.%4$I, sg.path || g.%4$I, g.%4$I = ANY(sg.path)
    FROM search_graph  sg
    JOIN %1$I.%2$I g ON g.%3$I = sg.%4$I
   WHERE NOT sg.cycle)
SELECT path
  FROM search_graph
 WHERE cycle
 LIMIT 1', TG_TABLE_SCHEMA, TG_TABLE_NAME, quote_ident(TG_ARGV[0]), quote_ident(TG_ARGV[1]))
INTO cycle
USING NEW;
IF cycle IS NULL
THEN
  RETURN NEW;
ELSE
   RAISE EXCEPTION 'Loop in %.% detected: %', TG_TABLE_SCHEMA, TG_TABLE_NAME, array_to_string(cycle, ' -> ');
END IF;

END
$func$ LANGUAGE plpgsql;

CREATE TRIGGER detect_cycle_after_update
 AFTER INSERT OR UPDATE ON node
   FOR EACH ROW EXECUTE PROCEDURE detect_cycle('id', 'parent_id');