PostgreSQL 8.3 中具有非空和唯一约束的增量字段

问题描述

我有一个表项目"，其中有一列位置".position 具有唯一且非空的约束.为了在位置 x 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;

这会导致违反唯一约束:

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，目前还不能选择使用 9.0.此外，UPDATE 语句不支持 ORDER BY 子句，以下也不起作用(仍然重复键违规):

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

另一个表，有多个唯一索引:

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)

我创建了一个以正确顺序更新位置值的过程:

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)

希望对你有帮助.

这篇关于PostgreSQL 8.3 中具有非空和唯一约束的增量字段的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！