Ok, aquí está mi intento de mantener las prioridades únicas y consecutivas. Implementado por una función disparador+. La parte difícil es evitar la recurrencia infinita que podría resultar de las actualizaciones desde dentro del disparador. Eso se soluciona con una bandera de suciedad/color, que hay que colocar dentro de la mesa. Su valor no es importante; sólo el cambio de la misma.
DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;
CREATE TABLE fruits
( id INTEGER NOT NULL PRIMARY KEY
, zname varchar NOT NULL
, priority INTEGER NOT NULL
, flipflag boolean NOT NULL default false
, CONSTRAINT unique_priority UNIQUE (priority) DEFERRABLE INITIALLY DEFERRED
);
INSERT INTO fruits(id,zname,priority) VALUES
(1 , 'Pear' ,4)
,(2 , 'Apple' ,2)
,(3 , 'Orange' ,1)
,(4 , 'Banana' ,3)
;
CREATE function shift_priority()
RETURNS TRIGGER AS $body$
BEGIN
UPDATE fruits fr
SET priority = priority +1
, flipflag = NOT flipflag -- alternating bit protocol ;-)
WHERE NEW.priority < OLD.priority
AND OLD.flipflag = NEW.flipflag -- redundant condition
AND fr.priority >= NEW.priority
AND fr.priority < OLD.priority
AND fr.id <> NEW.id -- exlude the initiating row
;
UPDATE fruits fr
SET priority = priority -1
, flipflag = NOT flipflag
WHERE NEW.priority > OLD.priority
AND OLD.flipflag = NEW.flipflag
AND fr.priority <= NEW.priority
AND fr.priority > OLD.priority
AND fr.id <> NEW.id
;
RETURN NEW;
END;
$body$
language plpgsql;
CREATE TRIGGER shift_priority
AFTER UPDATE OF priority ON fruits
FOR EACH ROW
WHEN (OLD.flipflag = NEW.flipflag AND OLD.priority <> NEW.priority)
EXECUTE PROCEDURE shift_priority()
;
UPDATE fruits
SET priority = 1
WHERE id=1;
RESULTADOS:
SELECT * FROM fruits ORDER BY id;
NOTICE: drop cascades to 2 other objects
DETAIL: drop cascades to table tmp.fruits
drop cascades to function tmp.shift_priority()
DROP SCHEMA
CREATE SCHEMA
SET
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "fruits_pkey" for table "fruits"
NOTICE: CREATE TABLE / UNIQUE will create implicit index "unique_priority" for table "fruits"
CREATE TABLE
INSERT 0 4
CREATE FUNCTION
CREATE TRIGGER
UPDATE 1
id | zname | priority | flipflag
----+--------+----------+----------
1 | Pear | 1 | f
2 | Apple | 3 | t
3 | Orange | 2 | t
4 | Banana | 4 | t
(4 rows)
