改编自之前的答案。当应用程序需要一个跳位顺序对于变量（读取：EAV 模型中的记录），它也可以是备用键（的一部分）。
- the
priority字段需要保持连续。 [这是跳位顺序]
- INSERT 时：优先级 >= 新记录的所有记录的优先级应增加
- 类似地：在 DELETE 上 -> 递减
- 如果记录的优先级因更新而改变，则旧优先级值和新优先级值之间的记录的优先级应向上或向下移动。
- to avoid recursive trigger invocation:
- 基于触发器的更新翻转
flipflag他们接触的任何记录。
- 他们测试
old.flipflag=new.flipflag检测real更新。 （非触发因素引起的）
-- Make some data
DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;
CREATE TABLE fruits
( id INTEGER NOT NULL PRIMARY KEY
, priority INTEGER NOT NULL
, flipflag boolean NOT NULL default false
, zname varchar NOT NULL
, 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)
,(5 , 'Peach' ,5)
;
-- Trigger functions for Insert/update/delete
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 function shift_down_priority()
RETURNS TRIGGER AS $body$
BEGIN
UPDATE fruits fr
SET priority = priority -1
, flipflag = NOT flipflag -- alternating bit protocol ;-)
WHERE fr.priority > OLD.priority
;
RETURN NEW;
END;
$body$
language plpgsql;
CREATE function shift_up_priority()
RETURNS TRIGGER AS $body$
BEGIN
UPDATE fruits fr
SET priority = priority +1
, flipflag = NOT flipflag -- alternating bit protocol ;-)
WHERE fr.priority >= NEW.priority
;
RETURN NEW;
END;
$body$
language plpgsql;
-- Triggers for Insert/Update/Delete
CREATE TRIGGER shift_priority_u
AFTER UPDATE OF priority ON fruits
FOR EACH ROW
WHEN (OLD.flipflag = NEW.flipflag AND OLD.priority <> NEW.priority)
EXECUTE PROCEDURE shift_priority()
;
CREATE TRIGGER shift_priority_d
AFTER DELETE ON fruits
FOR EACH ROW
EXECUTE PROCEDURE shift_down_priority()
;
CREATE TRIGGER shift_priority_i
BEFORE INSERT ON fruits
FOR EACH ROW
EXECUTE PROCEDURE shift_up_priority()
;
-- Do some I/U/D operations
\echo Pears are Okay
UPDATE fruits
SET priority = 1
WHERE id=1; -- 1,4
SELECT * FROM fruits ORDER BY priority;
\echo dont want bananas
DELETE FROM fruits WHERE id = 4;
SELECT * FROM fruits ORDER BY priority;
\echo We want Kiwis
INSERT INTO fruits(id,zname,priority) VALUES (4 , 'Kiwi' ,3) ;
SELECT * FROM fruits ORDER BY priority;
Result:
Pears are Okay
UPDATE 1
id | priority | flipflag | zname
----+----------+----------+--------
1 | 1 | f | Pear
3 | 2 | t | Orange
2 | 3 | t | Apple
4 | 4 | t | Banana
5 | 5 | f | Peach
(5 rows)
dont want bananas
DELETE 1
id | priority | flipflag | zname
----+----------+----------+--------
1 | 1 | f | Pear
3 | 2 | t | Orange
2 | 3 | t | Apple
5 | 4 | t | Peach
(4 rows)
We want Kiwis
INSERT 0 1
id | priority | flipflag | zname
----+----------+----------+--------
1 | 1 | f | Pear
3 | 2 | t | Orange
4 | 3 | f | Kiwi
2 | 4 | f | Apple
5 | 5 | f | Peach
(5 rows)
