PostgreSQL 8.3中具有非空且唯一约束的增量字段 [英] Increment field with not null and unique constraint in PostgreSQL 8.3

问题描述

我有一个带有列 position的表 items。位置具有唯一且非空的约束。为了在位置x处插入新行，我首先尝试增加后续项目的位置：

I have a table "items" with a column "position". position has a unique and not-null constraint. In order to insert a new row at position x I first try increment the positions of the subsequent items:

UPDATE items SET position = position + 1 WHERE position >= x;

这会导致独特的约束违反：

This results in a unique constraint violation:

ERROR:  duplicate key value violates unique constraint

问题似乎是PostgreSQL执行更新的顺序。 PostgreSQL中的唯一约束< 9.0不可延期，不幸的是，目前无法使用9.0。另外，UPDATE语句不支持ORDER BY子句，并且以下命令也不起作用（仍然重复键冲突）：

The problem seems to be the order in which PostgreSQL performs the updates. Unique constraints in PostgreSQL < 9.0 aren't deferrable and unfortunately using 9.0 is currently not an option. Also, the UPDATE statement doesn't support an ORDER BY clause and the following doesn't work, too (still duplicate key violation):

UPDATE items SET position = position + 1 WHERE id IN (
  SELECT id FROM items WHERE position >= x ORDER BY position DESC)

有人知道不涉及迭代代码中所有项目的解决方案吗？

Does somebody know a solution that doesn't involve iterating over all items in code?

推荐答案

另一个具有多个唯一索引的表：

Another table, with multiple unique index:

create table utest(id integer, position integer not null, unique(id, position));
test=# \d utest
      Table "public.utest"
  Column  |  Type   | Modifiers 
----------+---------+-----------
 id       | integer | 
 position | integer | not null
Indexes:
    "utest_id_key" UNIQUE, btree (id, "position")

一些数据：

insert into utest(id, position) select generate_series(1,3), 1;
insert into utest(id, position) select generate_series(1,3), 2;
insert into utest(id, position) select generate_series(1,3), 3;

test=# select * from utest order by id, position;
 id | position 
----+----------
  1 |        1
  1 |        2
  1 |        3
  2 |        1
  2 |        2
  2 |        3
  3 |        1
  3 |        2
  3 |        3
(9 rows)

我创建了一个程序，以正确的顺序更新头寸值：

I created a procedure that updates position values in the proper order:

create or replace function update_positions(i integer, p integer) 
  returns void as $$
declare
  temprec record;
begin
  for temprec in 
    select * 
      from utest u 
      where id = i and position >= p 
      order by position desc 
  loop
    raise notice 'Id = [%], Moving % to %', 
      i, 
      temprec.position, 
      temprec.position+1;

    update utest 
      set position = position+1 
      where position=temprec.position and id = i;
  end loop;
end;
$$ language plpgsql;

一些测试：

test=# select * from update_positions(1, 2);
NOTICE:  Id = [1], Moving 3 to 4
NOTICE:  Id = [1], Moving 2 to 3
 update_positions 
------------------

(1 row)

test=# select * from utest order by id, position;
 id | position 
----+----------
  1 |        1
  1 |        3
  1 |        4
  2 |        1
  2 |        2
  2 |        3
  3 |        1
  3 |        2
  3 |        3
(9 rows)

希望有帮助。

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