Prevent and/or detect cycles in postgres

问题

Assuming a schema like the following:

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

Further, let's assume the following data is present:

INSERT INTO node (name,parentid) VALUES
('A',NULL),
('B',1),
('C',1);

Is there a way to prevent cycles from being created? Example:

UPDATE node SET parentid = 2 WHERE id = 1;

This would create a cycle of 1->2->1->...

回答1:

Your trigger simplified and optimized, should be considerably faster:

CREATE OR REPLACE FUNCTION detect_cycle()
  RETURNS TRIGGER AS
$func$
BEGIN

IF EXISTS (
   WITH RECURSIVE search_graph(parentid, path, cycle) AS ( -- relevant columns
       -- check ahead, makes 1 step less
      SELECT g.parentid, ARRAY[g.id, g.parentid], (g.id = g.parentid)
      FROM   node g
      WHERE  g.id = NEW.id  -- only test starting from new row

      UNION ALL
      SELECT g.parentid, sg.path || g.parentid, g.parentid = ANY(sg.path)
      FROM   search_graph sg
      JOIN   node g ON g.id = sg.parentid
      WHERE  NOT sg.cycle
      )
   SELECT 1
   FROM   search_graph
   WHERE  cycle
   LIMIT  1  -- stop evalutation at first find
   )
THEN
   RAISE EXCEPTION 'Loop detected!';
ELSE
  RETURN NEW;
END IF;

END
$func$ LANGUAGE plpgsql;

You don't need dynamic SQL, you don't need to count, you don't need all the columns and you don't need to test the whole table for every single row.

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

An INSERT like this has to be prohibited, too:

INSERT INTO node (id, name,parentid) VALUES (8,'D',9), (9,'E',8);

回答2:

To answer my own question, I came up with a trigger that prevents this:

CREATE OR REPLACE FUNCTION detect_cycle() RETURNS TRIGGER AS
$func$
DECLARE
  loops INTEGER;
BEGIN
   EXECUTE 'WITH RECURSIVE search_graph(id, parentid, name, depth, path, cycle) AS (
        SELECT g.id, g.parentid, g.name, 1,
          ARRAY[g.id],
          false
        FROM node g
      UNION ALL
        SELECT g.id, g.parentid, g.name, sg.depth + 1,
          path || g.id,
          g.id = ANY(path)
        FROM node g, search_graph sg
        WHERE g.id = sg.parentid AND NOT cycle
)
SELECT count(*) FROM search_graph where cycle = TRUE' INTO loops;
IF loops > 0 THEN
  RAISE EXCEPTION 'Loop detected!';
ELSE
  RETURN NEW;
END IF;
END
$func$ LANGUAGE plpgsql;

CREATE TRIGGER detect_cycle_after_update
AFTER UPDATE ON node
FOR EACH ROW EXECUTE PROCEDURE detect_cycle();

So, if you try to create a loop, like in the question:

UPDATE node SET parentid = 2 WHERE id = 1;

You get an EXCEPTION:

ERROR:  Loop detected!

回答3:

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');

来源：https://stackoverflow.com/questions/26671612/prevent-and-or-detect-cycles-in-postgres